Re: [PATCH] x86/kvm: disable fast MMIO when running nested

Jason Wang
2018-01-26T02:42:10Z

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