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

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Message-ID: <5171cf03-4df7-2bd1-712e-c89717fa1ebc@amd.com>
Date:   Thu, 6 Sep 2018 12:50:05 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Borislav Petkov <bp@...e.de>
Cc:     brijesh.singh@....com, 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 09/06/2018 08:50 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.
> 
> Originally, my thought was that this would be a one-off case and the
> array could be freed directly in kvmclock_init(), e.g.:
> 


Please note that kvmclock_init() is called very early during the boot
process. We will not be able to use free_init_pages(...) so early.
Additionally, we also need to consider the bare-metal case -- which
will never call the kvmclock_init().



> static struct pvclock_vsyscall_time_info
> 	hv_clock_aux[HVC_AUX_ARRAY_SIZE] __decrypted __aligned(PAGE_SIZE);
> 
> ...
> 
> void __init kvmclock_init(void)
> {
> 	u8 flags;
> 
> 	if (!sev_active())
> 		free_init_pages("unused decrypted",
> 			(unsigned long)hv_clock_aux,
> 			(unsigned long)hv_clock_aux + sizeof(hv_clock_aux));
> 
>>