linux-kernel - Re: [PATCH 41/62] x86/sev-es: Handle MSR events

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Date:   Fri, 14 Feb 2020 08:59:39 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, hpa@...or.com, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 41/62] x86/sev-es: Handle MSR events

On 2/13/20 11:23 PM, Joerg Roedel wrote:
> Yes, investigating this is on the list for future optimizations (besides
> caching CPUID results). My idea is to use alternatives patching for
> this. But the exception handling is needed anyway because #VC
> exceptions happen very early already, basically the first thing after
> setting up a stack is calling verify_cpu(), which uses CPUID.

Ahh, bummer.  How does a guest know that it's running under SEV-ES?
What's the enumeration mechanism if CPUID doesn't "work"?

> The other reason is that things like MMIO and IOIO instructions can't be
> easily patched by alternatives. Those would work with the runtime
> checking you showed above, though.

Is there a reason we can't make a rule that you *must* do MMIO through
an accessor function so we *can* patch them?  I know random drivers
might break the rule, but are SEV-ES guests going to be running random
drivers?  I would think that they mostly if not all want to use virtio.