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

[Tom Lendacky]

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@xxxxxxxxxxxxxxx/