Re: [PATCH Part1 RFC v4 08/36] x86/sev: check the vmpl level

On Fri, Aug 13, 2021 at 08:13:20AM -0500, Brijesh Singh wrote:

During the firmware initialization the PSP requires that the VMPLEn is
set. See SNP firmware spec [1] section 8.6. To run the SNP guest you
*must* specify a VMPL level during the vCPU creation.

Yes, that's why I said "implicit VMPL level 0"! When you don't specify
it, it should implied as 0.

Right now that "enable" bit is useless as it is *forced* to be enabled.

I sincerely hope querying the VMPL level is going to be made
straight-forwaed in future versions.

I should not say its optional when we know from the SEV-SNP spec that
VMPLEn must be set to launch SEV-SNP guest. I will fix the description.

It probably wasn't required when that bit was invented - why would you
call it "enable" otherwise - but some decision later made it required,
I'd guess.

There is no easy way for a guest to query its VMPL level.

Yes, and there should be.

The VMPL level is set during the vCPU creation. The boot cpu is
created by the HV and thus its VMPL level is set by the HV. If HV
chooses a lower VMPL level for the boot CPU then Linux guest will
not be able to validate its memory because the PVALIDATE instruction
will cause #GP when the vCPU is running at !VMPL0. The patch tries to
detect the boot CPU VMPL level and terminate the boot.

I figured as much. All I don't like is the VMPL checking method.

If guest is not running at VMPL0 then step #2 will cause #GP.  The check
is prevent the #GP and terminate the boot early.

Yah, Tom helped me understand the design of the permission masks in the
RMP on IRC.

Thx.

-- 
Regards/Gruss,
    Boris.

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