Re: [PATCH v4 05/15] x86/sev: Use kernel provided SVSM Calling Areas

[Tom Lendacky]

On 5/8/24 14:58, Borislav Petkov wrote:
> On Wed, May 08, 2024 at 02:13:17PM -0500, Tom Lendacky wrote:
> > ok, maybe __perform_svsm_msr_protocol or such.
>
> We'll bikeshed it in the coming weeks.

:)

> > There's quite a bit of interaction so I'll make sure to prefix everything.
>
> Ack.
>
> > The paravirt versions of local_irq_save and local_irq_restore can't be used
> > as early as this routine is called.
>
> tglx says you should do native_local_irq_save()/.._restore() helpers
> just like the arch_local_irq_save()/..._restore() ones but use only
> native_ functions without the paravirt gunk.
>
> In a prepatch pls.

Will do.

> > > > +	struct svsm_call call = {};
> > > > +	int ret;
> > > > +	u64 pa;
> > > > +
> > > > +	/*
> > > > +	 * Record the SVSM Calling Area address (CAA) if the guest is not
> > > > +	 * running at VMPL0. The CA will be used to communicate with the
> > > > +	 * SVSM to perform the SVSM services.
> > > > +	 */
> > > > +	setup_svsm_ca(cc_info);
> > > > +
> > > > +	/* Nothing to do if not running under an SVSM. */
> > > > +	if (!vmpl)
> > > > +		return;
> > >
> > > You set up stuff above and now you bail out?
> >
> > setup_svsm_ca() is what sets the vmpl variable. So nothing will have been
> > setup if the VMPL is zero, in which case we don't continue on.
>
> You still assign
>
>          /*
>           * The CA is identity mapped when this routine is called, both by the
>           * decompressor code and the early kernel code.
>           */
>          boot_svsm_caa = (struct svsm_ca *)caa;
>          boot_svsm_caa_pa = caa;
>
> regardless of vmpl.

If we're not running at VMPL0 (based on the RMPADJUST check) and if the 
SVSM doesn't advertise a non-zero VMPL value, we will self-terminate. So 
those values are only set if we are not running at VMPL0 and the SVSM 
has provided a non-zero value to us.

I'm going to turn the function into a bool function so that the call 
becomes:

	if (!svsm_setup_caa(cc_info))
		return;

> I think you should assign those only when vmpl != 0.

I do. I think you're missing the RMPADJUST check that causes the 
function to return early if we're running at VMPL0.

> Otherwise the code is confusing.
>
> > > Judging by setup_svsm_ca() you don't really need that vmpl var but you
> > > can check
> > >
> > > 	if (!boot_svsm_caa)
> > > 		return;
> > >
> > > to determine whether a SVSM was detected.
> >
> > Yes, but the vmpl var will be used for attestation requests, sysfs, etc.
>
> I guess that comes later in the patchset...
>
> > > Huh, setup_svsm_ca() already assigned those...
> >
> > setup_svsm_ca() assigned the ones from the secrets page. The kernel now
> > switches to using its own CA.
>
> Comment pls.

There's a block comment above it all, but maybe it isn't clear enough. 
I'll rework it.

Thanks,
Tom

> Thx.