Re: [Xen-devel] [PATCH v3 02/34] x86/HVM: grow MMIO cache data size to 64 bytes

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 18/09/18 12:53, Jan Beulich wrote:
> This is needed before enabling any AVX512 insns in the emulator. Change
> the way alignment is enforced at the same time.
>
> Add a check that the buffer won't actually overflow, and while at it
> also convert the check for accesses to not cross page boundaries.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> v3: New.
>
> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -866,7 +866,18 @@ static int hvmemul_phys_mmio_access(
>      int rc = X86EMUL_OKAY;
>  
>      /* Accesses must fall within a page. */
> -    BUG_ON((gpa & ~PAGE_MASK) + size > PAGE_SIZE);
> +    if ( (gpa & ~PAGE_MASK) + size > PAGE_SIZE )
> +    {
> +        ASSERT_UNREACHABLE();
> +        return X86EMUL_UNHANDLEABLE;
> +    }
> +
> +    /* Accesses must not overflow the cache's buffer. */
> +    if ( size > sizeof(cache->buffer) )
> +    {
> +        ASSERT_UNREACHABLE();
> +        return X86EMUL_UNHANDLEABLE;
> +    }
>  
>      /*
>       * hvmemul_do_io() cannot handle non-power-of-2 accesses or
> --- a/xen/include/asm-x86/hvm/vcpu.h
> +++ b/xen/include/asm-x86/hvm/vcpu.h
> @@ -42,15 +42,14 @@ struct hvm_vcpu_asid {
>  };
>  
>  /*
> - * We may read or write up to m256 as a number of device-model
> + * We may read or write up to m512 as a number of device-model
>   * transactions.
>   */
>  struct hvm_mmio_cache {
>      unsigned long gla;
>      unsigned int size;
>      uint8_t dir;
> -    uint8_t pad[3]; /* make buffer[] long-aligned */
> -    uint8_t buffer[32];
> +    uint8_t buffer[64] __aligned(sizeof(long));

Don't we want it 16-byte aligned, rather than 8?

~Andrew

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