Re: [PATCH v3 3/3] KVM: SVM: Add 5-level page table support for SVM

[Paolo Bonzini]

On 18/08/21 19:32, Sean Christopherson wrote:
> On Wed, Aug 18, 2021, Wei Huang wrote:
> > When the 5-level page table is enabled on host OS, the nested page table
> > for guest VMs must use 5-level as well. Update get_npt_level() function
> > to reflect this requirement. In the meanwhile, remove the code that
> > prevents kvm-amd driver from being loaded when 5-level page table is
> > detected.
> >
> > Signed-off-by: Wei Huang <wei.huang2@xxxxxxx>
> > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>
> If this patch authored by Paolo, he needs to be attributed via From:.  If Paolo
> is a co-author, he needs a Co-developed-by:.  If either of those is true, your
> SOB needs to be last in the chain since you are the last handler of the patch.
> If neither is true, Paolo's SOB should be removed.

I didn't even remember writing this, but it's possible I pseudocoded in 
an email just like you did below.

> > -	return PT64_ROOT_4LEVEL;
> > +	bool la57 = (cr4_read_shadow() & X86_CR4_LA57) != 0;
> > +
> > +	return la57 ? PT64_ROOT_5LEVEL : PT64_ROOT_4LEVEL;
>
> Why obfuscate this?  KVM is completely hosed if pgtable_l5_enabled() doesn't
> match host CR4.  E.g.
>
> 	return pgtable_l5_enabled() ? PT64_ROOT_5LEVEL : PT64_ROOT_4LEVEL;

That also suggests the above pseudocoding scenario, where I'd be too 
lazy to look up the correct spelling of pgtable_l5_enabled().

Paolo

> >   #else
> >   	return PT32E_ROOT_LEVEL;
> >   #endif
> > @@ -462,11 +464,6 @@ static int has_svm(void)
> >   		return 0;
> >   	}
> >   
> > -	if (pgtable_l5_enabled()) {
> > -		pr_info("KVM doesn't yet support 5-level paging on AMD SVM\n");
> > -		return 0;
> > -	}
> > -
> >   	return 1;
> >   }
> >   
> > --
> > 2.31.1