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Re: [Xen-devel] [PATCH v2] libxc/x86: avoid overflow in CPUID APIC ID adjustments


  • To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Tue, 24 Sep 2019 18:58:43 +0100
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  • Cc: Juergen Gross <jgross@xxxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Tue, 24 Sep 2019 17:59:06 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 20/09/2019 14:54, Jan Beulich wrote:
> @@ -395,11 +395,20 @@ static void amd_xc_cpuid_policy(const st
>  
>      case 0x80000008:
>          /*
> -         * ECX[15:12] is ApicIdCoreSize: ECX[7:0] is NumberOfCores (minus 
> one).
> -         * Update to reflect vLAPIC_ID = vCPU_ID * 2.
> +         * ECX[15:12] is ApicIdCoreSize.
> +         * ECX[7:0] is NumberOfCores (minus one).
> +         * Update to reflect vLAPIC_ID = vCPU_ID * 2.  But make sure to avoid
> +         * - overflow,
> +         * - going out of sync with leaf 1 EBX[23:16],
> +         * - incrementing ApicIdCoreSize when it's zero (which changes the
> +         *   meaning of bits 7:0).
>           */
> -        regs[2] = ((regs[2] + (1u << 12)) & 0xf000u) |
> -                  ((regs[2] & 0xffu) << 1) | 1u;
> +        if ( (regs[2] & 0x7fu) < 0x7fu )

In attempting to rebase my series, there is a bug here.  It should be &
0xff otherwise the top bit isn't included in the comparison, and a value
of 128 will still be doubled.

~Andrew

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