Re: [PATCH -fixes] riscv: Implement flush_cache_vmap()

[Guo Ren]

On Tue, Aug 8, 2023 at 7:23 PM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
>
> Hey Dylan,
>
> Sorry I was busy debugging 6.5 failing to boot on the Unmatched (also
> TLB related, crazy everything converges to TLB issues lately :))
>
> On Fri, Aug 4, 2023 at 9:48 AM Dylan Jhong <dylan@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, Aug 03, 2023 at 11:48:36AM +0200, Alexandre Ghiti wrote:
> > > On Thu, Aug 3, 2023 at 11:25 AM Conor Dooley <conor.dooley@xxxxxxxxxxxxx> 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@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > > > > > > ---
> > > > > > >  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@xxxxxxxx/).
> > >
> > > 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
> > >
> > Hi Alex,
> >
> > Thank you for the detailed explanation. It is indeed undeniable that in certain
> > situations, there might be a possibility of repeated flushing TLB. But I think
> > there are some potential problem in flush_cache_vmap().
> >
> > In most case, vmap_range_noflush() and flush_cache_vmap() will appear at the same
> > time, so it should be no problem to choose one of them to do the TLB flush. But
> > flush_cache_vmap() does not cover all the places where apply_to_page_range()
> > appears (please correct me if I'm wrong), such as vmap_pfn()[1].
>
> That's a good catch, but shouldn't there be a flush_cache_vmap() in
> vmap_pfn()? What happens to architectures that implement
> flush_cache_vmap() and not arch_sync_kernel_mappings() (most of them)?
>
> >
> > The function you mentioned here, each will eventually call:
> >     vmap_range_noflush() -> arch_sync_kernel_mappings() -> TLB Flush
> >
> > As for the performance, because the current parameter of flush_tlb_page() needs to
> > pass *vma, we cannot pass in this parameter so we can only choose flush_tlb_all().
> > If it can be changed to flush_tlb_page() in the future, the performance should be improved.
>
> We should call only flush_tlb_kernel_range() on kernel addresses, so
> that won't be a problem.
Another idea for reference:
 1. keep vmalloc_fault()
 2. flush_tlb_page() for vmap_stack when creating kernel thread.

>
> >
> > [1]: https://elixir.bootlin.com/linux/v6.5-rc4/source/mm/vmalloc.c#L2977
> >
> > Best regards,
> > Dylan Jhong
> >
> > > > >
> > > > > [1]: http://lists.infradead.org/pipermail/linux-riscv/2023-August/037503.html
> > > >



-- 
Best Regards
 Guo Ren