Re: [PATCH RFC 02/39] KVM: x86/xen: intercept xen hypercalls if enabled

[Ankur Arora]

On 2020-12-02 12:03 a.m., David Woodhouse wrote:
> On Tue, 2020-12-01 at 21:19 -0800, Ankur Arora wrote:
> > > +             for (i = 0; i < PAGE_SIZE / sizeof(instructions); i++) {
> > > +                     *(u32 *)&instructions[1] = i;
> > > +                     if (kvm_vcpu_write_guest(vcpu,
> > > +                                              page_addr + (i * sizeof(instructions)),
> > > +                                              instructions, sizeof(instructions)))
> > > +                             return 1;
> > > +             }
> >
> > HYPERVISOR_iret isn't supported on 64bit so should be ud2 instead.
>
> Yeah, I got part way through typing that part but concluded it probably
> wasn't a fast path that absolutely needed to be emulated in the kernel.
>
> The VMM can inject the UD# when it receives the hypercall.

That would work as well but if it's a straight ud2 on the hypercall
page, wouldn't the guest just execute it when/if it does a
HYPERVISOR_iret?

Ankur


> I appreciate it *is* a guest-visible difference, if we're being really
> pedantic, but I don't think we were even going to be able to 100% hide
> the fact that it's not actually Xen.