linux-kernel - Re: [PATCH v6 4/5] x86/kvm: use __decrypted attribute in shared variables

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Date:   Mon, 10 Sep 2018 14:04:16 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v6 4/5] x86/kvm: use __decrypted attribute in shared
 variables

On Fri, Sep 07, 2018 at 12:57:29PM -0500, Brijesh Singh wrote:
> Commit: 368a540e0232 (x86/kvmclock: Remove memblock dependency)
> caused SEV guest regression.

When mentioning a commit in the commit message, put it on a separate
line, like this:

"Commit

  368a540e0232 (x86/kvmclock: Remove memblock dependency)

caused a SEV guest regression."

> When SEV is active, we map the shared

Use passive tone in your commit message: no "we", etc...

> variables (wall_clock and hv_clock_boot) with C=0 to ensure that both
> the guest and the hypervisor are able to access the data. To map the
> variables we use kernel_physical_mapping_init() to split the large pages,

"... to potentially split large pages used for that mapping... "

> but splitting large pages requires allocating a new PMD, which fails now
> that kvmclock initialization is called early during boot.

"... before the memblock allocator is initialized."

> Recently we added a special .data..decrypted section to hold the shared
> variables.

You don't really need that sentence.

> This section is mapped with C=0 early during boot. Use
> __decrypted attribute to put the wall_clock and hv_clock_boot in
> .data..decrypted section so that they are mapped with C=0.

"... so that they're mapped decrypted." Readers don't care about C=0
- they simply wanna know what C=0 represents, i.e., memory is not
encrypted.

With that:

Reviewed-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--