Messages in this thread Patch in this message | | | Date | Wed, 3 Apr 2024 21:37:30 +1000 | Subject | Re: [PATCH] arm64: tlb: Fix TLBI RANGE operand | From | Gavin Shan <> |
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On 4/3/24 18:58, Marc Zyngier wrote: > On Wed, 03 Apr 2024 07:49:29 +0100, > Gavin Shan <gshan@redhat.com> wrote: >> >> KVM/arm64 relies on TLBI RANGE feature to flush TLBs when the dirty >> bitmap is collected by VMM and the corresponding PTEs need to be >> write-protected again. Unfortunately, the operand passed to the TLBI >> RANGE instruction isn't correctly sorted out by commit d1d3aa98b1d4 >> ("arm64: tlb: Use the TLBI RANGE feature in arm64"). It leads to >> crash on the destination VM after live migration because some of the >> dirty pages are missed. >> >> For example, I have a VM where 8GB memory is assigned, starting from >> 0x40000000 (1GB). Note that the host has 4KB as the base page size. >> All TLBs for VM can be covered by one TLBI RANGE operation. However, >> I receives 0xffff708000040000 as the operand, which is wrong and the >> correct one should be 0x00007f8000040000. From the wrong operand, we >> have 3 and 1 for SCALE (bits[45:44) and NUM (bits943:39], only 1GB >> instead of 8GB memory is covered. >> >> Fix the macro __TLBI_RANGE_NUM() so that the correct NUM and TLBI >> RANGE operand are provided. >> >> Fixes: d1d3aa98b1d4 ("arm64: tlb: Use the TLBI RANGE feature in arm64") >> Cc: stable@kernel.org # v5.10+ >> Reported-by: Yihuang Yu <yihyu@redhat.com> >> Signed-off-by: Gavin Shan <gshan@redhat.com> >> --- >> arch/arm64/include/asm/tlbflush.h | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h >> index 3b0e8248e1a4..07c4fb4b82b4 100644 >> --- a/arch/arm64/include/asm/tlbflush.h >> +++ b/arch/arm64/include/asm/tlbflush.h >> @@ -166,7 +166,7 @@ static inline unsigned long get_trans_granule(void) >> */ >> #define TLBI_RANGE_MASK GENMASK_ULL(4, 0) >> #define __TLBI_RANGE_NUM(pages, scale) \ >> - ((((pages) >> (5 * (scale) + 1)) & TLBI_RANGE_MASK) - 1) >> + ((((pages) >> (5 * (scale) + 1)) - 1) & TLBI_RANGE_MASK) >> >> /* >> * TLB Invalidation > > This looks pretty wrong, by the very definition of the comment that's > just above: > > <quote> > /* > * Generate 'num' values from -1 to 30 with -1 rejected by the > * __flush_tlb_range() loop below. > */ > </quote> > > With your change, num can't ever be negative, and that breaks > __flush_tlb_range_op(): > > <quote> > num = __TLBI_RANGE_NUM(pages, scale); \ > if (num >= 0) { \ > addr = __TLBI_VADDR_RANGE(start >> shift, asid, \ > scale, num, tlb_level); \ > __tlbi(r##op, addr); \ > if (tlbi_user) \ > __tlbi_user(r##op, addr); \ > start += __TLBI_RANGE_PAGES(num, scale) << PAGE_SHIFT; \ > pages -= __TLBI_RANGE_PAGES(num, scale); \ > } \ > scale--; \ > </quote> > > We'll then shove whatever value we've found in the TLBI operation, > leading to unknown results instead of properly adjusting the scale to > issue a smaller invalidation. >
Marc, thanks for your review and comments.
Indeed, this patch is incomplete at least. I think we need __TLBI_RANGE_NUM() to return [-1 31] instead of [-1 30], to be consistent with MAX_TLBI_RANGE_PAGES. -1 will be rejected in the following loop. I'm not 100% sure if I did the correct calculation though.
/* * Generate 'num' values in range [-1 31], but -1 will be rejected * by the __flush_tlb_range() loop below. */ #define __TLBI_RANGE_NUM(pages, scale) \ ({ \ int __next = (pages) & (1ULL << (5 * (scale) + 6)); \ int __mask = ((pages) >> (5 * (scale) + 1)) & TLBI_RANGE_MASK; \ int __num = (((pages) >> (5 * (scale) + 1)) - 1) & \ TLBI_RANGE_MASK; \ (__next || __mask) ? __num : -1; \ })
Alternatively, we can also limit the number of pages to be invalidated from arch/arm64/kvm/hyp/pgtable.c::kvm_tlb_flush_vmid_range() because the maximal capacity is (MAX_TLBI_RANGE_PAGES - 1) instead of MAX_TLBI_RANGE_PAGES, as the comments for __flush_tlb_range_nosync() say.
- inval_pages = min(pages, MAX_TLBI_RANGE_PAGES); + inval_pages = min(pages, MAX_TLBI_RANGE_PAGES - 1);
static inline void __flush_tlb_range_nosync(...) { : /* * When not uses TLB range ops, we can handle up to * (MAX_DVM_OPS - 1) pages; * When uses TLB range ops, we can handle up to * (MAX_TLBI_RANGE_PAGES - 1) pages. */ if ((!system_supports_tlb_range() && (end - start) >= (MAX_DVM_OPS * stride)) || pages >= MAX_TLBI_RANGE_PAGES) { flush_tlb_mm(vma->vm_mm); return; } }
Please let me know which way is better.
> I think the problem is that you are triggering NUM=31 and SCALE=3, > which the current code cannot handle as per the comment above > __flush_tlb_range_op() (we can't do NUM=30 and SCALE=4, obviously). >
Yes, exactly.
> Can you try the untested patch below? > > > diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h > index 3b0e8248e1a4..b71a1cece802 100644 > --- a/arch/arm64/include/asm/tlbflush.h > +++ b/arch/arm64/include/asm/tlbflush.h > @@ -379,10 +379,6 @@ static inline void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch) > * 3. If there is 1 page remaining, flush it through non-range operations. Range > * operations can only span an even number of pages. We save this for last to > * ensure 64KB start alignment is maintained for the LPA2 case. > - * > - * Note that certain ranges can be represented by either num = 31 and > - * scale or num = 0 and scale + 1. The loop below favours the latter > - * since num is limited to 30 by the __TLBI_RANGE_NUM() macro. > */ > #define __flush_tlb_range_op(op, start, pages, stride, \ > asid, tlb_level, tlbi_user, lpa2) \ > @@ -407,6 +403,7 @@ do { \ > \ > num = __TLBI_RANGE_NUM(pages, scale); \ > if (num >= 0) { \ > + num += 1; \ > addr = __TLBI_VADDR_RANGE(start >> shift, asid, \ > scale, num, tlb_level); \ > __tlbi(r##op, addr); \ >
Thanks, but I don't think it's going to work. The loop will be running infinitely because the condition 'if (num >= 0)' can't be met when @pages is 0x200000 when @scale is 3/2/1/0 until @scale becomes negative and positive again, but @scale isn't in range [0 3]. I ported the chunk of code to user-space and I can see this with added printf() messages.
Thanks, Gavin
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