Re: [PATCH v8 08/40] x86/sev: Check the vmpl level

Mikolaj Lisik
2021-12-16T23:39:41Z

On Thu, Dec 16, 2021 at 12:24 PM Venu Busireddy
<venu.busireddy@oracle.com> wrote:
>
> On 2021-12-10 09:43:00 -0600, Brijesh Singh wrote:
> > Virtual Machine Privilege Level (VMPL) feature in the SEV-SNP architecture
> > allows a guest VM to divide its address space into four levels. The level
> > can be used to provide the hardware isolated abstraction layers with a VM.
> > The VMPL0 is the highest privilege, and VMPL3 is the least privilege.
> > Certain operations must be done by the VMPL0 software, such as:
> >
> > * Validate or invalidate memory range (PVALIDATE instruction)
> > * Allocate VMSA page (RMPADJUST instruction when VMSA=1)
> >
> > The initial SEV-SNP support requires that the guest kernel is running on
> > VMPL0. Add a check to make sure that kernel is running at VMPL0 before
> > continuing the boot. There is no easy method to query the current VMPL
> > level, so use the RMPADJUST instruction to determine whether the guest is
> > running at the VMPL0.
> >
> > Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> > ---
> > arch/x86/boot/compressed/sev.c | 34 ++++++++++++++++++++++++++++---
> > arch/x86/include/asm/sev-common.h | 1 +
> > arch/x86/include/asm/sev.h | 16 +++++++++++++++
> > 3 files changed, 48 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
> > index a0708f359a46..9be369f72299 100644
> > --- a/arch/x86/boot/compressed/sev.c
> > +++ b/arch/x86/boot/compressed/sev.c
> > @@ -212,6 +212,31 @@ static inline u64 rd_sev_status_msr(void)
> > return ((high << 32) | low);
> > }
> >
> > +static void enforce_vmpl0(void)
> > +{
> > + u64 attrs;
> > + int err;
> > +
> > + /*
> > + * There is no straightforward way to query the current VMPL level. The
> > + * simplest method is to use the RMPADJUST instruction to change a page
> > + * permission to a VMPL level-1, and if the guest kernel is launched at
> > + * a level <= 1, then RMPADJUST instruction will return an error.
>
> Perhaps a nit. When you say "level <= 1", do you mean a level lower than or
> equal to 1 semantically, or numerically?
>

+1 to this. Additionally I found the "level-1" confusing which I
interpreted as "level minus one".

Perhaps phrasing it as "level one", or "level=1" would be more explicit?