linux-kernel - Re: [PATCH] x86/sev-es: Do not unroll string I/O for SEV-ES guests

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Date:   Mon, 1 Feb 2021 12:34:31 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>, Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH] x86/sev-es: Do not unroll string I/O for SEV-ES guests

On 2/1/21 12:26 PM, Tom Lendacky wrote:
> From: Tom Lendacky <thomas.lendacky@....com>
> 
> Under the GHCB specification, SEV-ES guests can support string I/O. The
> current #VC handler contains this support, so remove the need to unroll
> kernel string I/O operations. This will reduce the number of #VC
> exceptions generated as well as the number VMEXITS for the guest.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
> ---
>   arch/x86/mm/mem_encrypt.c | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
> index c79e5736ab2b..d55ea77e1ca8 100644
> --- a/arch/x86/mm/mem_encrypt.c
> +++ b/arch/x86/mm/mem_encrypt.c
> @@ -474,9 +474,10 @@ void __init mem_encrypt_init(void)
>   	swiotlb_update_mem_attributes();
>   
>   	/*
> -	 * With SEV, we need to unroll the rep string I/O instructions.
> +	 * With SEV, we need to unroll the rep string I/O instructions,
> +	 * but SEV-ES supports them through the #VC handler.
>   	 */
> -	if (sev_active())
> +	if (sev_active() && !sev_es_active())
>   		static_branch_enable(&sev_enable_key);

This brings up a question. The name implies that this is a general SEV 
related key. However, it's currently only used for the string I/O 
operations. If further usage of this key is added in the future, then this 
would probably need to be split into two keys, the sev_enable_key and an 
sev_unroll_io_key.

Is it worth documenting that in the comment? Or should the key be renamed now?

Thanks,
Tom

>   
>   	print_mem_encrypt_feature_info();
> 
> base-commit: a7e0bdf1b07ea6169930ec42b0bdb17e1c1e3bb0
>