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: <4c6df918-0b47-e823-1141-82baa068828d@amd.com>
Date:   Thu, 6 Sep 2018 16:56:06 -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 03:39 PM, Sean Christopherson wrote:
> On Thu, Sep 06, 2018 at 03:20:46PM -0500, Brijesh Singh wrote:
>>
>>
>> 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.
> 
> Hmm, we'll need something otherwise 'NR_CPUS - HVC_BOOT_ARRAY_SIZE'
> will wrap and cause build errors.
> 


Right.

One option is we can hard-code the check for > 64, something like this:

#if defined(CONFIG_AMD_MEM_ENCRYPT) && NR_CPUS > 64
...
...
#endif

But this assumption will break if we ever add a new field in
struct pvclock_vsyscall_time_info. Hence I am not in favor of this.

Second option is, use KVM_MAX_VCPUS or NR_CPUS, something like this:

#ifdef CONFIG_AMD_MEM_ENCRYPT
  #define HVC_AUX_ARRAY_SIZE \
          PAGE_ALIGN(NR_CPUS * sizeof(struct pvclock_vsyscall_time_info))
...
#endif

In this case we will allocate few extra bytes which will get freed for
the non-SEV case anyways.