Re: [PATCH 1/3] accel/tcg: Fix unaligned stores to s390x low-address-protected lowcore

[David Hildenbrand <david@redhat.com>]

On 11.07.22 20:56, Ilya Leoshkevich wrote:
> If low-address-protection is active, unaligned stores to non-protected
> parts of lowcore lead to protection exceptions. The reason is that in
> such cases tlb_fill() call in store_helper_unaligned() covers
> [0, addr + size) range, which contains the protected portion of
> lowcore. This range is too large.
> 
> The most straightforward fix would be to make sure we stay within the
> original [addr, addr + size) range. However, if an unaligned access
> affects a single page, we don't need to call tlb_fill() in
> store_helper_unaligned() at all, since it would be identical to
> the previous tlb_fill() call in store_helper(), and therefore a no-op.
> If an unaligned access covers multiple pages, this situation does not
> occur.
> 
> Therefore simply skip TLB handling in store_helper_unaligned() if we
> are dealing with a single page.
> 
> Fixes: 2bcf018340cb ("s390x/tcg: low-address protection support")
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>  accel/tcg/cputlb.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/accel/tcg/cputlb.c b/accel/tcg/cputlb.c
> index f90f4312ea..a46f3a654d 100644
> --- a/accel/tcg/cputlb.c
> +++ b/accel/tcg/cputlb.c
> @@ -2248,7 +2248,7 @@ store_helper_unaligned(CPUArchState *env, target_ulong addr, uint64_t val,
>      const size_t tlb_off = offsetof(CPUTLBEntry, addr_write);
>      uintptr_t index, index2;
>      CPUTLBEntry *entry, *entry2;
> -    target_ulong page2, tlb_addr, tlb_addr2;
> +    target_ulong page1, page2, tlb_addr, tlb_addr2;
>      MemOpIdx oi;
>      size_t size2;
>      int i;
> @@ -2256,15 +2256,17 @@ store_helper_unaligned(CPUArchState *env, target_ulong addr, uint64_t val,
>      /*
>       * Ensure the second page is in the TLB.  Note that the first page
>       * is already guaranteed to be filled, and that the second page
> -     * cannot evict the first.
> +     * cannot evict the first.  An exception to this rule is PAGE_WRITE_INV
> +     * handling: the first page could have evicted itself.
>       */
> +    page1 = addr & TARGET_PAGE_MASK;
>      page2 = (addr + size) & TARGET_PAGE_MASK;
>      size2 = (addr + size) & ~TARGET_PAGE_MASK;
>      index2 = tlb_index(env, mmu_idx, page2);
>      entry2 = tlb_entry(env, mmu_idx, page2);
>  
>      tlb_addr2 = tlb_addr_write(entry2);
> -    if (!tlb_hit_page(tlb_addr2, page2)) {
> +    if (page1 != page2 && !tlb_hit_page(tlb_addr2, page2)) {
>          if (!victim_tlb_hit(env, mmu_idx, index2, tlb_off, page2)) {
>              tlb_fill(env_cpu(env), page2, size2, MMU_DATA_STORE,
>                       mmu_idx, retaddr);

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb