linux-kernel - Re: [PATCH v5 5/5] x86/kvm: Avoid dynamic allocation of pvclock data when SEV is active

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Date:   Thu, 6 Sep 2018 11:47:50 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     Borislav Petkov <bp@...e.de>, 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>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v5 5/5] x86/kvm: Avoid dynamic allocation of pvclock data
 when SEV is active

On Thu, Sep 06, 2018 at 01:37:50PM -0500, Brijesh Singh wrote:
> 
> 
> On 09/06/2018 09:18 AM, Sean Christopherson wrote:
> ....
> 
> >>>
> >>>So are we going to be defining a decrypted section for every piece of
> >>>machinery now?
> >>>
> >>>That's a bit too much in my book.
> >>>
> >>>Why can't you simply free everything in .data..decrypted on !SVE guests?
> >>
> >>That would prevent adding __decrypted to existing declarations, e.g.
> >>hv_clock_boot, which would be ugly in its own right.  A more generic
> >>solution would be to add something like __decrypted_exclusive to mark
> >>data that is used if and only if SEV is active, and then free the
> >>SEV-only data when SEV is disabled.
> >
> >Oh, and we'd need to make sure __decrypted_exclusive is freed when
> >!CONFIG_AMD_MEM_ENCRYPT, and preferably !sev_active() since the big
> >array is used only if SEV is active.  This patch unconditionally
> >defines hv_clock_dec but only frees it if CONFIG_AMD_MEM_ENCRYPT=y &&
> >!mem_encrypt_active().
> >
> 
> Again we have to consider the bare metal scenario while doing this. The
> aux array you proposed will be added in decrypted section only when
> CONFIG_AMD_MEM_ENCRYPT=y.  If CONFIG_AMD_MEM_ENCRYPT=n then nothng
> gets put in .data.decrypted section. At the runtime, if memory
> encryption is active then .data.decrypted_hvclock will contains useful
> data.
> 
> The __decrypted attribute in "" when CONFIG_AMD_MEM_ENCRYPT=n.

Right, but won't the data get dumped into the regular .bss in that
case, i.e. needs to be freed?