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: <9126c92f-ad3b-a5b6-ac44-416405489a7e@amd.com>
Date:   Thu, 6 Sep 2018 15:20:46 -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 02:47 PM, Sean Christopherson wrote:
...

>>
>> 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.
> 
> I just realized that we'll try to create a bogus array if 'NR_CPUS <=
> HVC_BOOT_ARRAY_SIZE'.  A bit ugly, but we could #ifdef away both that
> and CONFIG_AMD_MEM_ENCRYPT=n in a single shot, e.g.:
> 
> #if defined(CONFIG_AMD_MEM_ENCRYPT) && NR_CPUS > HVC_BOOT_ARRAY_SIZE
> #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 __aligned(PAGE_SIZE);
> #endif
> 

The HVC_BOOT_ARRAY_SIZE macro uses sizeof(..) and to my understanding
the sizeof operators are not allowed in '#if'. Anyway, I will try to see
if it can be used, if not then I will stick to CONFIG_AMD_MEM_ENCRYPT
check.

-Brijesh