linux-kernel - Re: [PATCH v1 1/2] x86/sev: Use per-CPU PSC structure in prep for unaccepted memory support

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Message-ID: <9da5204f-5c8f-b47b-bcc6-d68ca63db08d@amd.com>
Date:   Fri, 29 Jul 2022 09:25:20 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        "H. Peter Anvin" <hpa@...or.com>,
        Michael Roth <michael.roth@....com>,
        Joerg Roedel <jroedel@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v1 1/2] x86/sev: Use per-CPU PSC structure in prep for
 unaccepted memory support



On 7/29/22 09:18, Dave Hansen wrote:
> On 7/29/22 07:01, Tom Lendacky wrote:
>> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
>> index c05f0124c410..1f7f6205c4f6 100644
>> --- a/arch/x86/kernel/sev.c
>> +++ b/arch/x86/kernel/sev.c
>> @@ -104,6 +104,15 @@ struct sev_es_runtime_data {
>>   	 * is currently unsupported in SEV-ES guests.
>>   	 */
>>   	unsigned long dr7;
>> +
>> +	/*
>> +	 * Page State Change structure for use when accepting memory or when
>> +	 * changing page state. Interrupts are disabled when using the structure
>> +	 * but an NMI could still be raised, so use a flag to indicate when the
>> +	 * structure is in use and use the MSR protocol in these cases.
>> +	 */
>> +	struct snp_psc_desc psc_desc;
>> +	bool psc_active;
>>   };
> 
> This thing:
> 
> struct snp_psc_desc {
>          struct psc_hdr hdr;
>          struct psc_entry entries[VMGEXIT_PSC_MAX_ENTRY];
> } __packed;
> 
> is 16k, right?  Being per-cpu, this might eat up a MB or two of memory
> on a big server?

It's just under 2K, 2,032 bytes.

> 
> Considering that runtime acceptance is already single-threaded[1] *and*
> there's a fallback method, why not just have a single copy of this
> guarded by a single lock?

This function is called for more than just memory acceptance. It's also 
called for any changes from or to private or shared, which isn't 
single-threaded.

Thanks,
Tom

> 
> 1.
> https://lore.kernel.org/all/20220614120231.48165-10-kirill.shutemov@linux.intel.com/