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

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Date:   Wed, 2 Dec 2020 10:20:17 -0800
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        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 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.
>