Re: [PATCH v8 13/40] x86/kernel: Make the bss.decrypted section shared in RMP table

Venu Busireddy
2022-01-06T20:17:05Z

On 2022-01-06 13:06:13 -0600, Tom Lendacky wrote:
> On 1/6/22 11:40 AM, Venu Busireddy wrote:
> > On 2022-01-05 15:39:22 -0600, Brijesh Singh wrote:
> > >
> > >
> > > On 1/5/22 2:27 PM, Dave Hansen wrote:
> > > > On 1/5/22 11:52, Brijesh Singh wrote:
> > > > > > >           for (; vaddr < vaddr_end; vaddr += PMD_SIZE) {
> > > > > > > +            /*
> > > > > > > +             * When SEV-SNP is active then transition the
> > > > > > > page to shared in the RMP
> > > > > > > +             * table so that it is consistent with the page
> > > > > > > table attribute change.
> > > > > > > +             */
> > > > > > > +            early_snp_set_memory_shared(__pa(vaddr),
> > > > > > > __pa(vaddr), PTRS_PER_PMD);
> > > > > >
> > > > > > Shouldn't the first argument be vaddr as below?
> > > > >
> > > > > Nope, sme_postprocess_startup() is called while we are fixing the
> > > > > initial page table and running with identity mapping (so va == pa).
> > > >
> > > > I'm not sure I've ever seen a line of code that wanted a comment so badly.
> > >
> > > The early_snp_set_memory_shared() call the PVALIDATE instruction to clear
> > > the validated bit from the BSS region. The PVALIDATE instruction needs a
> > > virtual address, so we need to use the identity mapped virtual address so
> > > that PVALIDATE can clear the validated bit. I will add more comments to
> > > clarify it.
> >
> > Looking forward to see your final comments explaining this. I can't
> > still follow why, when PVALIDATE needs the virtual address, we are doing
> > a __pa() on the vaddr.
>
> It's because of the phase of booting that the kernel is in. At this point,
> the kernel is running in identity mapped mode (VA == PA). The
> __start_bss_decrypted address is a regular kernel address, e.g. for the
> kernel I'm on it is 0xffffffffa7600000. Since the PVALIDATE instruction
> requires a valid virtual address, the code needs to perform a __pa() against
> __start_bss_decrypted to get the identity mapped virtual address that is
> currently in place.

Perhaps my confusion stems from the fact that __pa(x) is defined either
as "((unsigned long ) (x))" (for the cases where paddr and vaddr are
same), or as "__phys_addr((unsigned long )(x))", where a vaddr needs to
be converted to a paddr. If the paddr and vaddr are same in our case,
what exactly is the _pa(vaddr) doing to the vaddr?

Venu