Re: [PATCH 10/11] KVM: MMU: fix detecting misaligned accessed

Xiao Guangrong
2011-07-27T10:08:39Z

On 07/27/2011 05:15 PM, Avi Kivity wrote:
> On 07/26/2011 02:31 PM, Xiao Guangrong wrote:
>> Sometimes, we only modify the last one byte of a pte to update status bit,
>> for example, clear_bit is used to clear r/w bit in linux kernel and 'andb'
>> instruction is used in this function, in this case, kvm_mmu_pte_write will
>> treat it as misaligned access, and the shadow page table is zapped
>>
>> @@ -3597,6 +3597,14 @@ static bool detect_write_misaligned(struct kvm_mmu_page *sp, gpa_t gpa,
>>
>> offset = offset_in_page(gpa);
>> pte_size = sp->role.cr4_pae ? 8 : 4;
>> +
>> + /*
>> + * Sometimes, the OS only writes the last one bytes to update status
>> + * bits, for example, in linux, andb instruction is used in clear_bit().
>> + */
>> + if (sp->role.level == 1&& !(offset& (pte_size - 1))&& bytes == 1)
>> + return false;
>> +
>
> Could be true for level > 1, no?
>

In my origin mind, i thought one-byte-instruction is usually used to update the last pte,
but we do better remove this restriction. Will fix it in the next version, thanks!