Re: [PATCH 3/3] KVM: x86: improve reexecute_instruction

Xiao Guangrong
2012-11-28T23:16:50Z

On 11/29/2012 06:40 AM, Xiao Guangrong wrote:
> On 11/29/2012 05:57 AM, Marcelo Tosatti wrote:
>> On Wed, Nov 28, 2012 at 10:59:35PM +0800, Xiao Guangrong wrote:
>>> On 11/28/2012 10:12 PM, Gleb Natapov wrote:
>>>> On Tue, Nov 27, 2012 at 11:30:24AM +0800, Xiao Guangrong wrote:
>>>>> On 11/27/2012 06:41 AM, Marcelo Tosatti wrote:
>>>>>
>>>>>>>
>>>>>>> - return false;
>>>>>>> +again:
>>>>>>> + page_fault_count = ACCESS_ONCE(vcpu->kvm->arch.page_fault_count);
>>>>>>> +
>>>>>>> + /*
>>>>>>> + * if emulation was due to access to shadowed page table
>>>>>>> + * and it failed try to unshadow page and re-enter the
>>>>>>> + * guest to let CPU execute the instruction.
>>>>>>> + */
>>>>>>> + kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
>>>>>>> + emulate = vcpu->arch.mmu.page_fault(vcpu, cr3, PFERR_WRITE_MASK, false);
>>>>>>
>>>>>> Can you explain what is the objective here?
>>>>>>
>>>>>
>>>>> Sure. :)
>>>>>
>>>>> The instruction emulation is caused by fault access on cr3. After unprotect
>>>>> the target page, we call vcpu->arch.mmu.page_fault to fix the mapping of cr3.
>>>>> if it return 1, mmu can not fix the mapping, we should report the error,
>>>>> otherwise it is good to return to guest and let it re-execute the instruction
>>>>> again.
>>>>>
>>>>> page_fault_count is used to avoid the race on other vcpus, since after we
>>>>> unprotect the target page, other cpu can enter page fault path and let the
>>>>> page be write-protected again.
>>>>>
>>>>> This way can help us to detect all the case that mmu can not be fixed.
>>>>>
>>>> Can you write this in a comment above vcpu->arch.mmu.page_fault()?
>>>
>>> Okay, if Marcelo does not object this way. :)
>>
>> I do object, since it is possible to detect precisely the condition by
>> storing which gfns have been cached.
>>
>> Then, Xiao, you need a way to handle large read-only sptes.
>
> Sorry, Marcelo, i am still confused why read-only sptes can not work
> under this patch?
>
> The code after read-only large spte is is:
>
> + if ((level > PT_PAGE_TABLE_LEVEL &&
> + has_wrprotected_page(vcpu->kvm, gfn, level)) ||
> + mmu_need_write_protect(vcpu, gfn, can_unsync)) {
> pgprintk("%s: found shadow page for %llx, marking ro\n",
> __func__, gfn);
> ret = 1;
>
> It return 1, then reexecute_instruction return 0. It is the same as without
> readonly large-spte.

Ah, wait, There is a case, the large page located at 0-2M, the 0-4K is used as a
page-table (e.g. PDE), and the guest want to write the memory located at 5K which
should be freely written. This patch can return 0 for both current code and readonly
large spte.

I need to think it more.