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

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Date:   Fri, 26 Jan 2018 04:49:41 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Liran Alon <liran.alon@...cle.com>, vkuznets@...hat.com,
        x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] x86/kvm: disable fast MMIO when running nested

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.

-- 
MST