linux-kernel - Re: [PATCH v4 4/4] x86/kvm: use __decrypted attribute in shared variables

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Message-ID: <7b004581-dfb0-1418-1252-0860e5e93cd0@amd.com>
Date:   Wed, 5 Sep 2018 13:09:45 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
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>,
        Borislav Petkov <bp@...e.de>, "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v4 4/4] x86/kvm: use __decrypted attribute in shared
 variables

Hi Sean,


On 09/04/2018 05:32 PM, Sean Christopherson wrote:

...

>>   
>>   	/* Use the static page for the first CPUs, allocate otherwise */
>> -	if (cpu < HVC_BOOT_ARRAY_SIZE)
>> +	if (cpu < HVC_BOOT_ARRAY_SIZE) {
>>   		p = &hv_clock_boot[cpu];
>> -	else
>> -		p = kzalloc(sizeof(*p), GFP_KERNEL);
>> +	} else {
>> +		int rc;
>> +		unsigned int sz = sizeof(*p);
>> +
>> +		if (sev_active())
>> +			sz = PAGE_ALIGN(sz);
> 
> Hmm, again we're wasting a fairly sizable amount of memory since each
> CPU is doing a separate 4k allocation.  What if we defined an auxilary
> array in __decrypted to be used for cpus > HVC_BOOT_ARRAY_SIZE when
> SEV is active?  struct pvclock_vsyscall_time_info is 32 bytes so we
> could handle the max of 8192 CPUs with 256kb of data (252kb if you
> subtract the pre-existing 4k page), i.e. the SEV case wouldn't need
> additional memory beyond the 2mb page that's reserved for __decrypted.
> The non-SEV case could do free_kernel_image_pages() on the unused
> array (which would need to be page sized) so it wouldn't waste memory.


Sure, we could use unused memory range from .data..decrypted section
for cpu > HVC_BOOT_ARRAY_SIZE when SEV is active. I will rework the
patch.

-Brijesh