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

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Date:   Wed, 02 Dec 2020 08:03:02 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Ankur Arora <ankur.a.arora@...cle.com>,
        Joao Martins <joao.m.martins@...cle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH RFC 02/39] KVM: x86/xen: intercept xen hypercalls if
 enabled

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.

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.

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