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

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Message-ID: <YQSAVo0CXUKHXdLF@google.com>
Date:   Fri, 30 Jul 2021 22:42:30 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, Eric Biederman <ebiederm@...ssion.com>,
        kexec@...ts.infradead.org, Joerg Roedel <jroedel@...e.de>,
        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

On Wed, Jul 21, 2021, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.de>
> 
> Handle CLFLUSH instruction to MMIO memory in the #VC handler. The
                               ^
			       |- emulated

> instruction is ignored by the handler, as the Hypervisor is
> responsible for cache management of emulated MMIO memory.
> 
> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
>  arch/x86/kernel/sev-shared.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
> index a7a0793c4f98..682fa202444f 100644
> --- a/arch/x86/kernel/sev-shared.c
> +++ b/arch/x86/kernel/sev-shared.c
> @@ -632,6 +632,15 @@ static enum es_result vc_handle_mmio_twobyte_ops(struct ghcb *ghcb,
>  	long *reg_data;
>  
>  	switch (insn->opcode.bytes[1]) {
> +		/* CLFLUSH */
> +	case 0xae:
> +		/*
> +		 * Ignore CLFLUSHes - those go to emulated MMIO anyway and the
> +		 * hypervisor is responsible for cache management.

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.
		 */

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

> +		 */
> +		ret = ES_OK;
> +		break;
> +
>  		/* MMIO Read w/ zero-extension */
>  	case 0xb6:
>  		bytes = 1;
> -- 
> 2.31.1
>