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 14:46:26 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     brijesh.singh@....com, 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 09/06/2018 02:24 PM, Brijesh Singh wrote:

...

>>>
>>> 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?
>>
> 
> 
> Yes, the auxiliary array will dumped into the regular .bss when
> CONFIG_AMD_MEM_ENCRYPT=n. Typically it will be few k, I am not
> sure if its worth complicating the code to save those extra memory.
> Most of the distro's have CONFIG_AMD_MEM_ENCRYPT=y anyways.

We can use #ifdef CONFIG_AMD_MEM_ENCRYPT around hv_clock_aux definition
so that it gets defined

Something like this:

#ifdef CONFIG_AMD_MEM_ENCRYPT
/* The auxilary array will be used when SEV is active */
#define HVC_AUX_ARRAY_SIZE \
         PAGE_ALIGN((NR_CPUS - HVC_BOOT_ARRAY_SIZE) * \
                 sizeof(struct pvclock_vsyscall_time_info))
static struct pvclock_vsyscall_time_info
                         hv_clock_aux[HVC_AUX_ARRAY_SIZE] __decrypted_aux;
#endif