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Message-ID: <684bd330949528ecd352d4a381165c2681c0bae9.camel@infradead.org>
Subject: Re: [PATCH RFC 02/39] KVM: x86/xen: intercept xen hypercalls if
 enabled
From:   David Woodhouse <dwmw2@infradead.org>
To:     Ankur Arora <ankur.a.arora@oracle.com>,
        Joao Martins <joao.m.martins@oracle.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org
Date:   Wed, 02 Dec 2020 08:03:02 +0000
In-Reply-To: <240b82b3-9621-8b08-6d37-75da6a61b3ce@oracle.com>
References: <20190220201609.28290-1-joao.m.martins@oracle.com>
         <20190220201609.28290-3-joao.m.martins@oracle.com>
         <b56f763e6bf29a65a11b7a36c4d7bfa79b2ec1b2.camel@infradead.org>
         <240b82b3-9621-8b08-6d37-75da6a61b3ce@oracle.com>
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On Tue, 2020-12-01 at 21:19 -0800, Ankur Arora wrote:
> > +             for (i =3D 0; i < PAGE_SIZE / sizeof(instructions); i++) =
{
> > +                     *(u32 *)&instructions[1] =3D i;
> > +                     if (kvm_vcpu_write_guest(vcpu,
> > +                                              page_addr + (i * sizeof(=
instructions)),
> > +                                              instructions, sizeof(ins=
tructions)))
> > +                             return 1;
> > +             }
>=20
> 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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