Messages in this thread | | | From | Alexandre Ghiti <> | Date | Thu, 3 Aug 2023 11:58:41 +0200 | Subject | Re: [PATCH -fixes] riscv: Implement flush_cache_vmap() |
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On Thu, Aug 3, 2023 at 11:48 AM Alexandre Ghiti <alexghiti@rivosinc.com> wrote: > > On Thu, Aug 3, 2023 at 11:25 AM Conor Dooley <conor.dooley@microchip.com> wrote: > > > > On Thu, Aug 03, 2023 at 05:14:15PM +0800, dylan wrote: > > > On Sun, Jul 30, 2023 at 01:08:17AM -0400, Guo Ren wrote: > > > > On Tue, Jul 25, 2023 at 9:22 AM Alexandre Ghiti <alexghiti@rivosinc.com> wrote: > > > > > > > > > > The RISC-V kernel needs a sfence.vma after a page table modification: we > > > > > used to rely on the vmalloc fault handling to emit an sfence.vma, but > > > > > commit 7d3332be011e ("riscv: mm: Pre-allocate PGD entries for > > > > > vmalloc/modules area") got rid of this path for 64-bit kernels, so now we > > > > > need to explicitly emit a sfence.vma in flush_cache_vmap(). > > > > > > > > > > Note that we don't need to implement flush_cache_vunmap() as the generic > > > > > code should emit a flush tlb after unmapping a vmalloc region. > > > > > > > > > > Fixes: 7d3332be011e ("riscv: mm: Pre-allocate PGD entries for vmalloc/modules area") > > > > > Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com> > > > > > --- > > > > > arch/riscv/include/asm/cacheflush.h | 4 ++++ > > > > > 1 file changed, 4 insertions(+) > > > > > > > > > > diff --git a/arch/riscv/include/asm/cacheflush.h b/arch/riscv/include/asm/cacheflush.h > > > > > index 8091b8bf4883..b93ffddf8a61 100644 > > > > > --- a/arch/riscv/include/asm/cacheflush.h > > > > > +++ b/arch/riscv/include/asm/cacheflush.h > > > > > @@ -37,6 +37,10 @@ static inline void flush_dcache_page(struct page *page) > > > > > #define flush_icache_user_page(vma, pg, addr, len) \ > > > > > flush_icache_mm(vma->vm_mm, 0) > > > > > > > > > > +#ifdef CONFIG_64BIT > > > > > +#define flush_cache_vmap(start, end) flush_tlb_kernel_range(start, end) > > > > Sorry, I couldn't agree with the above in a PIPT cache machine. It's > > > > not worth for. > > > > > > > > It would reduce the performance of vmap_pages_range, > > > > ioremap_page_range ... API, which may cause some drivers' performance > > > > issues when they install/uninstall memory frequently. > > > > > > > > > > Hi All, > > > > > > I think functional correctness should be more important than system performance > > > in this case. The "preventive" SFENCE.VMA became necessary due to the RISC-V > > > specification allowing invalidation entries to be cached in the TLB. > > > > We are at -rc4 and this stuff is broken. Taking the bigger hammer, which > > can be reverted later when a more targeted fix shows up, to make sure > > that v6.5 doesn't end up broken, sounds rather prudent. Otherwise, the > > original commit should probably be reverted. > > The original commit that removed vmalloc_fault() is required, handling > vmalloc faults in the page fault path is not possible (see the links > in the description of 7d3332be011e and the example that I gave in the > thread https://lore.kernel.org/linux-riscv/dc26625b-6658-c078-76d2-7e975a04b1d4@ghiti.fr/).
Actually the problem was there before this commit (Dylan has the issue in 6.1), so no faulty commit to revert :)
> > I totally agree with Dylan that we'll work (I'm currently working on > that) on the performance side of the problem in the next release, we > need correctness and for that we need a preventive global sfence.vma > as we have no means (for now) to distinguish between uarch that cache > or not invalid entries. > > > > > > The problem[1] we are currently encountering is caused by not updating the TLB > > > after the page table is created, and the solution to this problem can only be > > > solved by updating the TLB immediately after the page table is created. > > > > > > There are currently two possible approaches to flush TLB: > > > 1. Flush TLB in flush_cache_vmap() > > > 2. Flush TLB in arch_sync_kernel_mappings() > > > > > > But I'm not quite sure if it's a good idea to operate on the TLB inside flush_cache_vmap(). > > > The name of this function indicates that it should be related to cache operations, maybe > > > it would be more appropriate to do TLB flush in arch_sync_kernel_mappings()? > > TLDR: The downsides to implementing arch_sync_kernel_mappings() > instead of flush_cache_vmap(): > > - 2 global flushes for vunmap instead of 1 for flush_cache_vmap() > - flushes the tlb in the noflush suffixed functions so it prevents any > flush optimization (ie: a loop of vmap_range_noflush() without flush > and then a final flush afterwards) > > So I'd favour the flush_cache_vmap() implementation which seems > lighter. powerpc does that > https://elixir.bootlin.com/linux/latest/source/arch/powerpc/include/asm/cacheflush.h#L27 > (but admits that it may not be the right place) > > Here is the long story (my raw notes): > > * arch_sync_kernel_mappings() is called from: > - _apply_to_page_range(): would only emit global sfence.vma if vmalloc > addresses, I guess that's ok. > - __vunmap_range_noflush(): it is noted here > https://elixir.bootlin.com/linux/latest/source/mm/vmalloc.c#L406 that > any caller must call flush_tlb_kernel_range(). Then the implementation > of arch_sync_kernel_mappings() would result in 2 global tlb flushes. > - vmap_range_noflush(): does not fit well with the noflush() suffix. > > * flush_cache_vmap() is called from: > - kasan_populate_vmalloc(): legit since it bypasses vmap api (but > called right a apply_to_page_range() so your patch would work here) > - kmsan_vunmap_range_noflush(): called twice for the mappings kmsan > establishes and flush_tlb_kernel_range() must be called afterwards => > 3 global tlb flushes but the 3 are needed as they target different > addresses. Implementing only arch_sync_kernel_mappings() would result > in way more global flushes (see the loop here > https://elixir.bootlin.com/linux/latest/source/mm/kmsan/hooks.c#L151 > where __vmap_pages_range_noflush() would result in more > flush_tlb_all()) > - kmsan_vmap_pages_range_noflush(): here we would flush twice, but > same thing for the arch_sync_kernel_mappings() implementation. > - ioremap_page_range(): legit, same as arch_sync_kernel_mappings() > implementation. > - vmap_pages_range(): legit, same as arch_sync_kernel_mappings() implementation. > > Let me know what you think! > > Alex > > > > > > > [1]: http://lists.infradead.org/pipermail/linux-riscv/2023-August/037503.html > >
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