Basic Virtual Memory Implementation Fixes & Improvements #165

nemecad · 2025-09-04T10:53:57Z

This PR addresses some previous feedback, most notably:

Set-associative TLB (machine::TLB): Implements a set-associative Translation Lookaside Buffer (TLB) frontend over physical memory, handling virtual to physical translation, flush, and replacement policy.
Pluggable Replacement Policies (machine::TLBPolicy): Abstract TLB replacement policy interface & implementations (RAND, LRU, LFU, PLRU) for set-associative tables.
SV32 Page-Table Walker (machine::PageTableWalker): Performs multi-level page-table walks (SV32) in memory to resolve a virtual address to a physical one.
Sv32Pte Bitfield Helpers (sv32.h): SV32-specific definitions: page-table entry (PTE) bitfields, shifts/masks, and PTE to physical address helpers.
VirtualAddress (virtual_address.h): Lightweight VirtualAddress wrapper offering raw access, alignment checks, arithmetic, and comparisons.
Add supervisor CSRs and sstatus handling: supervisor CSRs (sstatus, stvec, sscratch, sepc, scause, stval, satp) and a write handler that presents sstatus as a masked view of mstatus so supervisor-visible bits stay in sync.
Store current privilege level in CoreState: tracking of the hart's current privilege level in CoreState so exception/return handling and visualization can read/update it from the central CoreState structure.

Tests:

Add SV32 page-table + TLB integration tests: a set of small assembly tests that exercise the SV32 page-table walker, SATP enablement and the new TLB code. The tests create a root page table and map a virtual page at 0xC4000000, then exercise several scenarios. The tests verify page-table walker behaviour, SATP switching and TLB caching/flush logic. Tests were written based on the consultation.

UI Components:

Show current privilege level in core state view:

Virtual memory configuration to NewDialog:

TLB visualization and statistics dock:

VM toggle and "As CPU" memory access view:

src/gui/CMakeLists.txt

src/gui/dialogs/new/newdialog.cpp

src/gui/windows/tlb/tlbview.cpp

src/gui/windows/tlb/tlbview.h

src/machine/CMakeLists.txt

src/machine/machine.cpp

src/machine/machine.h

src/machine/machineconfig.cpp

src/machine/memory/frontend_memory.cpp

src/machine/memory/virtual/sv32.h

jdupak · 2025-09-28T18:14:08Z

I am getting this weird zoom.

jdupak · 2025-09-28T18:14:47Z

Notice that address sanitizer is failing in CI.

jdupak · 2025-09-28T18:15:58Z

src/gui/windows/tlb/tlbview.h

+        void tlb_update(unsigned way, unsigned set, bool valid, unsigned asid, quint64 vpn, quint64 phys, bool write);
+
+private:
+    const machine::TLB *tlb;


Who owns this pointer?

jdupak · 2025-09-28T18:17:38Z

src/machine/memory/tlb/tlb.h

+#include <cstdint>
+
+namespace machine {
+enum TLBType { PROGRAM, DATA };


Why does LTB need to know this?

ppisa · 2025-09-29T08:51:35Z

@jdupak thanks for review of interfacing to the memory model architecture.

ppisa · 2025-09-29T08:51:45Z

From my side, the changes to the processor pipeline diagram has been applied directly to the SVG files (src/gui/windows/coreview/schemas), but current design uses DRAW.IO source (extras/core_graphics) as the authoritative source of the pipeline visualization and SVGs are generated from this file. So the commit with SVG change should include extras/core_graphics/diagram.drawio change as well or extras/core_graphics/diagram.drawio change should be commit before SVG files regeneration commit. In long term, I would lean to single SVG file with tags for conditional rendering, but we have not got to that state yet and current solution implemented by @jdupak is based on DRAW.IO and exports controlled by tagging (some documentation there docs/developer/coreview-graphics/using-drawio-diagram.md).

ppisa · 2025-09-29T08:55:46Z

For memory view, I would not complicate it with Show virtual checkbox. I would use only switching between As CPU (VMA), Cached and Raw.

nemecad · 2025-10-05T19:38:26Z

@ppisa @jdupak Thank you for your detailed feedback. I appreciate it and have made some changes based on your review. I would be grateful for any further feedback.

This resolves the incorrect zoom behavior.

Add tracking of the hart's current privilege level to the core state so code handling exceptions/returns and visualization can read/update it from the central CoreState structure.

Add supervisor CSRs (sstatus, stvec, sscratch, sepc, scause, stval, satp) and a write handler that presents sstatus as a masked view of mstatus so supervisor-visible bits stay in sync.

jdupak · 2025-10-19T14:49:24Z

tests/cli/virtual_memory/template/program.S

There is a typo in the dir name

jdupak · 2025-10-19T14:50:16Z

tests/cli/virtual_memory/template/program.S

There is no cmake logic to run these tests. I think we want to run them as cli tests.

Thank you for the comment. I’ve added the CMake logic to run these as CLI tests in the commit 7a204cf.

Add privilege level mapping to the GUI so the current hart privilege (UNPRIV, SUPERV, HYPERV, MACHINE) is displayed in core state visualization.

Extend NewDialog with controls for virtual memory setup, including TLB number of sets, associativity, and replacement policy.

Introduce new components for displaying and tracking TLB state similar to cache. TLBViewBlock and TLBAddressBlock render per-set and per-way TLB contents, updated on tlb_update signals. TLBViewScene assembles these views based on associativity. TLBDock integrates into the GUI, showing hit/miss counts, memory accesses, stall cycles, hit rate, and speed improvement, with live updates from the TLB.

Introduce an "As CPU (VMA)" access option in the cached access selector to render memory contents as observed by the CPU through the frontend interface.

jdupak · 2025-10-19T16:16:39Z

I pushed some slight edits. Barring the issue with new tests not being run I am fine with merging this.

ppisa · 2025-10-29T15:14:09Z

I am going through the code. I have one overall remark, that there are lot of formatting changes included in functional changes. I am not reluctant to formatting changes even that I think that sometimes formatting left by human to align for example some case lines into columns etc. has some value. But formatting changes unrelated to the functional changes make review harder. So I would keep with patches as they are but I would suggest to separate formatting, even over all later modified files in series, separate from functional changes.

ppisa · 2025-10-29T15:51:19Z

I am do not like is_mmio_region() and bypass_mmio() concept. The peripherals accesses should go through regular address translation. It is responsibility of the OS to map regions related to I/O into virtual address space of kernel and or even user application, i.e. for mmap() like accesses.

As for enabling cache for accesses there is a hack in the QtRvSim which enforces next uncached region

Cache::Cache
    uncached_start(0xf0000000_addr)
    uncached_last(0xfffffffe_addr)

In the longer term, cacheability should be controlled from page tables. But PBMT (Page-Based Memory Types) are supported only for Sv39 and bigger translation configurations, see Chapter 14. "Svpbmt" Extension for Page-Based Memory Types

Mode	Value	Requested Memory Attributes
PMA	0	None
NC	1	Non-cacheable, idempotent, weakly-ordered (RVWMO), main memory
IO	2	Non-cacheable, non-idempotent, strongly-ordered (I/O ordering), I/O
-	3	Reserved for future standard use

But the physical region marked to skip caching in cache implementation (current state) should be enough for now.

ppisa · 2025-10-29T16:17:04Z

Not so critical for now, but should be solved in the longer time perspective. SRET can be executed even in M mode. So the type of the return should be propagated to the control_state->exception_return in the Core::memory(const ExecuteInterstage &dt). It is question if to add signal which goes through all stages (more readable) or to use bit from instruction for local decode of the type. MRET should not be allowed in system mode. In general, I think that current version does not mark system level instructions and access to the system and machine mode CSRs as invalid in U mode. So some masking would be required on the decode level in future. Some more flags needs to be added into enum InstructionFlags to allow that checking and flags_to_check and it should then be updated on mode transition.

The behavior of xRET instructions is described in 3.1.6.1. Privilege and Global Interrupt-Enable Stack in mstatus register. When SRET is executed in M mode then it executes the same as in the S mode but it
should clear MPRV=0. This is to allow emulate some system level operations in machine level code.

ppisa · 2025-10-29T16:56:29Z

It seems that TLBs are updated from the start of the system. The TLB and its updates should be enable only when root register is set. And they should not be updated in M mode at all.

jdupak · 2025-10-29T18:49:40Z

There is one actual issue from CI: you cannot use ftruncate. It fails compilation on Win.

jdupak · 2025-10-30T10:41:10Z

There is one actual issue from CI: you cannot use ftruncate. It fails compilation on Win.

Never mind, this is broken on master. I will fix that. It does not block this PR.

Add a set of small assembly tests that exercise the SV32 page-table walker, SATP enablement and the new TLB code. The tests create a root page table and map a virtual page at 0xC4000000, then exercise several scenarios. The tests verify page-table walker behaviour, SATP switching and TLB caching/flush logic. Tests were written based on the consultation.

Restructure the virtual memory test directory to align with the rest of the tests, and add CMake logic for running them.

Ensure that TLBs are only updated when the root register is set, and disable TLB updates while running in Machine mode.

Decode MRET/SRET/URET in the decode stage, carry the return type through interstage registers, and pass it to ControlState::exception_return in the memory stage. Mark returns illegal if they request a higher privilege (e.g., MRET in S-mode). When returning from M to a less-privileged mode, clear MPRV per the privileged spec.

jdupak self-requested a review September 28, 2025 17:17

jdupak reviewed Sep 28, 2025

View reviewed changes

nemecad force-pushed the feature/sv32-vm-tlb-ptw-cleanup branch 5 times, most recently from 0bb04d1 to ca4300b Compare October 5, 2025 19:16

ppisa mentioned this pull request Oct 10, 2025

CLI: bugfix and improvements #166

Closed

jdupak and others added 4 commits October 19, 2025 16:29

Fix deprecation warnings for custom literals

2602c1e

GUI: fix zoom scaling issue

82aaf4e

This resolves the incorrect zoom behavior.

Machine: store current privilege level in CoreState

6c6c7da

Add tracking of the hart's current privilege level to the core state so code handling exceptions/returns and visualization can read/update it from the central CoreState structure.

Machine: add supervisor CSRs and sstatus handling

363141c

Add supervisor CSRs (sstatus, stvec, sscratch, sepc, scause, stval, satp) and a write handler that presents sstatus as a masked view of mstatus so supervisor-visible bits stay in sync.

jdupak force-pushed the feature/sv32-vm-tlb-ptw-cleanup branch from ca4300b to a6cbf71 Compare October 19, 2025 14:48

jdupak reviewed Oct 19, 2025

View reviewed changes

tests/cli/virtual_memory/template/program.S

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jdupak Oct 19, 2025

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There is a typo in the dir name

jdupak reviewed Oct 19, 2025

View reviewed changes

jdupak and others added 5 commits October 19, 2025 18:11

Machine: add TLB with policies and add SV32 page-table walker

2d5db0a

GUI: show current privilege level in core state view

d7ff20e

Add privilege level mapping to the GUI so the current hart privilege (UNPRIV, SUPERV, HYPERV, MACHINE) is displayed in core state visualization.

GUI: add virtual memory configuration to NewDialog

bdfdea1

Extend NewDialog with controls for virtual memory setup, including TLB number of sets, associativity, and replacement policy.

GUI: add "As CPU (VMA)" memory access view

14153a3

Introduce an "As CPU (VMA)" access option in the cached access selector to render memory contents as observed by the CPU through the frontend interface.

jdupak force-pushed the feature/sv32-vm-tlb-ptw-cleanup branch from a6cbf71 to bc32933 Compare October 19, 2025 16:13

nemecad force-pushed the feature/sv32-vm-tlb-ptw-cleanup branch from 7a204cf to b8728d1 Compare November 2, 2025 14:11

Tests: add cmake logic to run virtual memory tests as CLI tests

0ae766f

Restructure the virtual memory test directory to align with the rest of the tests, and add CMake logic for running them.

nemecad force-pushed the feature/sv32-vm-tlb-ptw-cleanup branch from b8728d1 to 0ae766f Compare November 2, 2025 14:15

Machine: gate TLB updates by root register and privilege mode

442a091

Ensure that TLBs are only updated when the root register is set, and disable TLB updates while running in Machine mode.

nemecad force-pushed the feature/sv32-vm-tlb-ptw-cleanup branch from c846a9e to 442a091 Compare November 2, 2025 16:54

Basic Virtual Memory Implementation Fixes & Improvements #165

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Basic Virtual Memory Implementation Fixes & Improvements #165

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