linux-kernel - Re: [PATCH 11/12] x86/sev: Handle CLFLUSH MMIO events

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Date:   Sat, 31 Jul 2021 09:17:37 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Joerg Roedel <joro@...tes.org>, x86@...nel.org,
        Eric Biederman <ebiederm@...ssion.com>,
        kexec@...ts.infradead.org, hpa@...or.com,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Martin Radev <martin.b.radev@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 11/12] x86/sev: Handle CLFLUSH MMIO events

Hi Sean,

On Fri, Jul 30, 2021 at 10:42:30PM +0000, Sean Christopherson wrote:
> On Wed, Jul 21, 2021, Joerg Roedel wrote:
> This wording can be misread as "the hypervisor is responsible for _all_ cache
> management".  Maybe just:
> 
> 		/*
> 		 * Ignore CLFLUSHes - the hyperivsor is responsible for cache
> 		 * management of emulated MMIO.
> 		 */

Right, will update the comment, thanks.

> Side topic, out of curisoity, what's mapping/accessing emulated MMIO as non-UC?

The CLFLUSHes happen when the kexec'ed kernel maps the VGA framebuffer
as unencrypted. Initially it is mapped encrypted and before re-mapping
the kernel flushes the range from the caches.

I have not investigated why this doesn't happen on the first boot,
though.

Regards,

	Joerg