Re: [PATCH Part2 RFC v4 33/40] KVM: SVM: Add support to handle MSR based Page State Change VMGEXIT

[Brijesh Singh]

On 7/19/21 1:55 PM, Sean Christopherson wrote:
> I've no objection to using PSC for enums and whatnot, and I'll happily defer to
> Boris for functions in the core kernel and guest, but for KVM I'd really like to
> spell out the name for the two or so main handler functions.

Noted.


> > Maybe I am missing something, I am not able to follow 'guest won't be able
> > to convert a 2mb chunk back to a 2mb large page'. The page-size used inside
> > the guest have to relationship with the RMP/NPT page-size. e.g, a guest can
> > validate the page range as a 4k and still map the page range as a 2mb or 1gb
> > in its pagetable.
>
> The proposed code walks KVM's TDP and adjusts the RMP level to be the min of the
> guest+host levels.  Once KVM has installed a 4kb TDP SPTE, that walk will find
> the 4kb TDP SPTE and thus operate on the RMP at a 4kb granularity.  To allow full
> restoration of 2mb PTE+SPTE+RMP, KVM needs to zap the 4kb SPTE(s) at some point
> to allow rebuilding a 2mb SPTE.

Ah I see. In that case, SNP firmware provides a command 
"SNP_PAGE_UNMASH" that can be used by the hypervisor to combines the 
multiple 4k entry into a single 2mb without affecting the validation.

-Brijesh