Re: [PATCH] x86/mm: fix pg_level_to_pfn for 5-level paging mode.

[Kalra, Ashish]

On 10/14/2023 3:47 AM, Ingo Molnar wrote:
> * Ashish Kalra <Ashish.Kalra@xxxxxxx> wrote:
>
> > From: Ashish Kalra <ashish.kalra@xxxxxxx>
> >
> > Add support for 5-level paging in pg_level_to_pfn().
> >
> > Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> > ---
> >   arch/x86/mm/mem_encrypt_amd.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/arch/x86/mm/mem_encrypt_amd.c b/arch/x86/mm/mem_encrypt_amd.c
> > index 6faea41e99b6..45914568596a 100644
> > --- a/arch/x86/mm/mem_encrypt_amd.c
> > +++ b/arch/x86/mm/mem_encrypt_amd.c
> > @@ -267,6 +267,10 @@ static unsigned long pg_level_to_pfn(int level, pte_t *kpte, pgprot_t *ret_prot)
> >   		pfn = pud_pfn(*(pud_t *)kpte);
> >   		prot = pud_pgprot(*(pud_t *)kpte);
> >   		break;
> > +	case PG_LEVEL_512G:
> > +		pfn = p4d_pfn(*(p4d_t *)kpte);
> > +		prot = p4d_pgprot(*(p4d_t *)kpte);
> > +		break;
>
> Mind explaining what happens if we don't have this fix, how you found it,
> which users are expected to be affected by it, whether that's the full
> known extent of 5-level paging support deficiencies in mem_encrypt_amd.c,foud 
> etc?

It was found during working on SNP guest kexec support, this helper 
function is used during early boot stage for setting of C-bit on the 
PTEs and additionally need this same helper for switching shared pages 
to private during guest kexec, so i believe this patch is needed.

Thanks,
Ashish

> In addition to such patches being much easier on the eyes, it also helps
> maintainers prioritize patches.
>
> Thanks,
>
> 	Ingo