linux-kernel - Re: [PATCH RFC dontapply] kvm_para: add mmio word store hypercall

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Date:	Mon, 26 Mar 2012 13:30:38 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Joerg Roedel <joerg.roedel@....com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC dontapply] kvm_para: add mmio word store hypercall

On Mon, Mar 26, 2012 at 12:16:14PM +0200, Avi Kivity wrote:
> On 03/26/2012 12:08 PM, Michael S. Tsirkin wrote:
> >
> > > > +		gpa = hc_gpa(vcpu, a1, a2);
> > > > +		if (!write_mmio(vcpu, gpa, 2, &a0) && run) {
> > > 
> > > What's this && run thing?
> >
> > I'm not sure - copied this from another other place in emulation:
> > arch/x86/kvm/x86.c:4953:                if (!write_mmio(vcpu, gpa, 2, &a0) && run)
> >
> > I assumed there's some way to trigger emulation while VCPU does not run.
> > No?
> 
> Not the way you initialize run above.

Thanks for pointing this out, I'll drop the test.

> >
> > > 
> > > > +			run->exit_reason = KVM_EXIT_MMIO;
> > > > +			run->mmio.phys_addr = gpa;
> > > > +			memcpy(run->mmio.data, &a0, 2);
> > > > +			run->mmio.len = 2;
> > > > +			run->mmio.is_write = 1;
> > > > +                        r = 0;
> > > > +		}
> > > > +		goto noret;
> > > 
> > > What if the address is in RAM?
> > > Note the guest can't tell if a piece of memory is direct mapped or
> > > implemented as mmio.
> >
> > True but doing hypercalls for memory which can be
> > mapped directly is bad for performance - it's
> > the reverse of what we are trying to do here.
> 
> It's bad, but the guest can't tell.
> 
> Suppose someone implements virtio in hardware and we pass it through to
> a guest.  It should continue working, no?

Why would we want hypercalls then?

As I see it, virtio device would have a capability
that tells the guest to use hypercalls for access.
An actual PCI device won't expose this capability,
as would a device on a host which lacks the hypercall.


> > The intent is to use this for virtio where we can explicitly let the
> > guest know whether using a hypercall is safe.
> >
> > Acceptable?  What do you suggest?
> 
> It's iffy.

Question is, do we want a bunch of dead code sitting there
just in case? And what are the chances it'll work correctly
when we need it to?

> What's the performance gain from this thing?

I'll test and post separately.

> 
> -- 
> error compiling committee.c: too many arguments to function
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