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

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Date:	Mon, 26 Mar 2012 12:29:21 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Avi Kivity <avi@...hat.com>, 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 12:08:29PM +0200, Michael S. Tsirkin wrote:
> On Mon, Mar 26, 2012 at 11:21:58AM +0200, Avi Kivity wrote:
> > On 03/26/2012 12:05 AM, Michael S. Tsirkin wrote:
> > > We face a dilemma: IO mapped addresses are legacy,
> > > so, for example, PCI express bridges waste 4K
> > > of this space for each link, in effect limiting us
> > > to 16 devices using this space.
> > >
> > > Memory is supposed to replace them, but memory
> > > exits are much slower than PIO because of the need for
> > > emulation and page walks.
> > >
> > > As a solution, this patch adds an MMIO hypercall with
> > > the guest physical address + data.
> > >
> > > I did test that this works but didn't benchmark yet.
> > >
> > > TODOs:
> > > This only implements a 2 bytes write since this is
> > > the minimum required for virtio, but we'll probably need
> > > at least 1 byte reads (for ISR read).
> > > We can support up to 8 byte reads/writes for 64 bit
> > > guests and up to 4 bytes for 32 ones - better limit
> > > to 4 bytes for everyone for consistency, or support
> > > the maximum that we can?
> > 
> > Let's support the maximum we can.
> > 
> > >  
> > >  static int handle_invd(struct kvm_vcpu *vcpu)
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index 9cbfc06..7bc00ae 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -4915,7 +4915,9 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
> > >  
> > >  int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> > >  {
> > > +	struct kvm_run *run = vcpu->run;
> > >  	unsigned long nr, a0, a1, a2, a3, ret;
> > > +	gpa_t gpa;
> > >  	int r = 1;
> > >  
> > >  	if (kvm_hv_hypercall_enabled(vcpu->kvm))
> > > @@ -4946,12 +4948,24 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> > >  	case KVM_HC_VAPIC_POLL_IRQ:
> > >  		ret = 0;
> > >  		break;
> > > +	case KVM_HC_MMIO_STORE_WORD:
> > 
> > HC_MEMORY_WRITE
> 
> Do we really want guests to access random memory this way though?
> Even though it can, how about HC_PCI_MEMORY_WRITE to stress the intended
> usage?
> See also discussion below.
> 
> > > +		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)
> 
What git tree is this from? I think that's the one you added.

> I assumed there's some way to trigger emulation while VCPU does not run.
> No?
> 
> > 
> > > +			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.
> 
> 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?
> 
> > 
> > -- 
> > error compiling committee.c: too many arguments to function
> --
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--
			Gleb.
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