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: <9b7ba5a6-5b2d-206d-46f0-32e0dbd2af70@amd.com>
Date:   Thu, 6 Sep 2018 13:50:21 -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 v5 5/5] x86/kvm: Avoid dynamic allocation of pvclock data
 when SEV is active



On 09/06/2018 09:07 AM, Sean Christopherson wrote:
...

>> +
>> +/* This should cover upto 512 VCPUS (first 64 are covered by hv_clock_boot[]). */
>> +#define HVC_DECRYPTED_ARRAY_SIZE \
>> +	((PAGE_SIZE * 7)  / sizeof(struct pvclock_vsyscall_time_info))
> 
> I think we can define the size relative to NR_CPUS rather than picking
> an arbitrary number of pages, maybe with a BUILD_BUG_ON to make sure
> the total size won't require a second 2mb page for __decrpyted.
> 
> #define HVC_DECRYPTED_ARRAY_SIZE  \
> 	PAGE_ALIGN((NR_CPUS - HVC_BOOT_ARRAY_SIZE) * \
> 		   sizeof(struct pvclock_vsyscall_time_info))
> 

Sure works for me.

>> +static struct pvclock_vsyscall_time_info
>> +			hv_clock_dec[HVC_DECRYPTED_ARRAY_SIZE] __decrypted_hvclock;
>> +
>>   static inline struct pvclock_vcpu_time_info *this_cpu_pvti(void)
>>   {
>>   	return &this_cpu_read(hv_clock_per_cpu)->pvti;
>> @@ -267,10 +274,19 @@ static int kvmclock_setup_percpu(unsigned int cpu)
>>   		return 0;
>>   
>>   	/* 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 {
>> +		/*
>> +		 * When SEV is active, use the static pages from
>> +		 * .data..decrypted_hvclock section. The pages are already
>> +		 * mapped with C=0.
>> +		 */
>> +		if (sev_active())
>> +			p = &hv_clock_dec[cpu - HVC_BOOT_ARRAY_SIZE];
>> +		else
>> +			p = kzalloc(sizeof(*p), GFP_KERNEL);
>> +	}
> 
> Personal preference, but I think an if-elif-else with a single block
> comment would be easier to read.


I can do with that. thanks for your feedback.


> 
> 	/*
> 	 * Blah blah blah
> 	 */
> 	if (cpu < HVC_BOOT_ARRAY_SIZE)
> 		p = &hv_clock_boot[cpu];
> 	else if (sev_active())
> 		p = &hv_clock_dec[cpu - HVC_BOOT_ARRAY_SIZE];
> 	else
> 		p = kzalloc(sizeof(*p), GFP_KERNEL);
>