Re: [PATCH v1 6/7] kvm/x86: Hyper-V SynIC message slot pending clearing at SINT ack

[Paolo Bonzini <pbonzini@redhat.com>]

On 26/11/2015 10:06, Andrey Smetanin wrote:
> 
> 
> On 11/25/2015 08:14 PM, Paolo Bonzini wrote:
>>
>>
>> On 25/11/2015 17:55, Andrey Smetanin wrote:
>>>>
>>>> +    gpa = synic->msg_page & PAGE_MASK;
>>>> +    page = kvm_vcpu_gfn_to_page(vcpu, gpa >> PAGE_SHIFT);
>>>> +    if (is_error_page(page)) {
>>>> +        vcpu_err(vcpu, "Hyper-V SynIC can't get msg page, gpa
>>>> 0x%llx\n",
>>>> +             gpa);
>>>> +        return;
>>>> +    }
>>>> +    msg_page = kmap_atomic(page);
>>>
>>> But the message page is not being pinned, is it?
>>>
>>> Actually I don't know anything about pinning.
>>> Is it pinning against page swapping ?
>>
>> Yes.  Unless the page is pinned, kmap_atomic can fail.
> kmap_atomic() can't fail for a valid page struct. Does
> kvm_vcpu_gfn_to_page() can provide invalid page(swapped page) struct
> which may pass is_error_page(page) check but can leads to incorrect
> behavior inside kmap_atomic()?

No, you're right.  Nevermind, I was confused because I thought you
needed kmap_atomic rather than kmap.  Here using kmap_atomic is just an
optimization, so it's okay.  (If you needed kmap_atomic, the problem
would have been that kvm_vcpu_gfn_to_page() can sleep).

In patch 7/7 you're also not in atomic context, so kvm_vcpu_gfn_to_page
is okay.

Shouldn't have reviewed the patch when tired. :)

Then the patches look good, I think.  With a testcase I can try them out
and hopefully merge them for Linux 4.5 / QEMU 2.6.

Paolo