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Subject: Re: [PATCH] x86/kvm: disable fast MMIO when running nested
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Liran Alon <liran.alon@oracle.com>, vkuznets@redhat.com,
        x86@kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <8170a43b-7c7a-ef61-7a53-a5495fc8c6fd@redhat.com>
Date:   Fri, 26 Jan 2018 11:21:27 +0800
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On 2018年01月26日 10:49, Michael S. Tsirkin wrote:
> On Fri, Jan 26, 2018 at 10:41:58AM +0800, Jason Wang wrote:
>>
>> On 2018年01月26日 01:11, Michael S. Tsirkin wrote:
>>> On Thu, Jan 25, 2018 at 09:49:22AM -0500, Paolo Bonzini wrote:
>>>>>> Michael and Jason, any progress on implementing a fast virtio mechanism
>>>>>> that doesn't rely on undefined behavior?
>>>>>>
>>>>>> (Encode writing instruction length into last 4 bits of MMIO address,
>>>>>>     side-channel say that accesses to the MMIO area always use certain
>>>>>>     instruction length, use hypercall, ...)
>>>>>>
>>>>>> Thanks.
>>>>> No progress from my side. But we can use PIO for virtio 1.0 and it's
>>>>> faster than fast MMIO (qemu supports modern pio notification bar, we can
>>>>> make it as default). It looks to me that neither encoding nor hypercall
>>>>> will work for real hardware virtio device.
>>>> Encoding the instruction length would work, the h/w virtio devices would
>>>> just ignore it.  But... it is really ugly.
>>>>
>>>> Using PIO would be a small step backwards for PCIe.  As long as the device
>>>> only needs *one* notification register (either MMIO or PIO) to initialize
>>>> successfully, it's okay.  Then if there is no PIO space you'd just fall back
>>>> to the slower MMIO notification.
>>>>
>>>> Paolo
>>> A bigger issue for PIO is it's causing exits for hw devices.
>>>
>>>
>> Just to make sure I understand. For exits you mean vmexit? I believe MMIO
>> will cause vmexit too.
>>
>> Thanks
> Not with an assigned device where the PTE is marked as present, it
> won't.
>

So in this case, assigned device can just provide MMIO bar.

Thanks