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Re: [Xen-devel] [PATCH v2] libxc/x86: avoid overflow in CPUID APIC ID adjustments
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <JBeulich@xxxxxxxx>
- Date: Wed, 25 Sep 2019 05:56:40 +0000
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- Cc: Juergen Gross <JGross@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 25 Sep 2019 05:58:08 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Thread-index: AQHVb7rdga0Iv13OHE+wW09XJH/8hqc7JCNCgADIXQA=
- Thread-topic: [PATCH v2] libxc/x86: avoid overflow in CPUID APIC ID adjustments
On 24.09.2019 19:58, Andrew Cooper wrote:
> 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.
Oh, indeed, thanks for spotting. Will send a fixup patch once in the
office, unless you did so already.
Jan
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