lkml.org 
[lkml]   [2012]   [Mar]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [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
    > --
    > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    > the body of a message to majordomo@vger.kernel.org
    > More majordomo info at http://vger.kernel.org/majordomo-info.html
    > Please read the FAQ at http://www.tux.org/lkml/

    --
    Gleb.


    \
     
     \ /
      Last update: 2012-03-26 12:31    [W:0.032 / U:88.480 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site