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Re: [Xen-devel] [PATCH v9 08/23] x86emul: support AVX512PF insns


  • To: Jan Beulich <JBeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Thu, 4 Jul 2019 15:44:28 +0100
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  • Cc: Wei Liu <wl@xxxxxxx>, RogerPau Monne <roger.pau@xxxxxxxxxx>
  • Delivery-date: Thu, 04 Jul 2019 14:44:36 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 01/07/2019 12:20, Jan Beulich wrote:
> +        /* Clear untouched parts of the mask value. */
> +        n = 1 << (4 - ((b & 1) | evex.w));
> +        op_mask &= (1 << n) - 1;
> +
> +        for ( i = 0; rc == X86EMUL_OKAY && op_mask; ++i )
> +        {
> +            signed long idx = b & 1 ? index.qw[i] : index.dw[i];
> +
> +            if ( !(op_mask & (1 << i)) )
> +                continue;

It occurs from my recent foray into UBSAN that op_mask is 64 bits wide,
although it looks like n can be at maximum 16 in this specific case.

If nothing else, using (1u << 1) would reduce the size of the UBSAN
build, but I expect we're soon going to have subtle bugs when we get to
the int8 instructions.

Are there current S/G instructions which can take 32 iterations?

~Andrew

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