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

Brijesh Singh
2021-12-17T22:19:32Z

On 12/16/21 5:39 PM, Mikolaj Lisik wrote:
> 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?

Its numerically, please see the AMD APM vol 3.

Here is the snippet from the APM RMPAJUST.

IF (TARGET_VMPL <= CURRENT_VMPL)  // Only permissions for numerically

EAX = FAIL_PERMISSION                // higher VMPL can be modified

EXIT

> +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?
>
Sure, I will make it clear that its target vmpl level 1 and not (target
level - 1).

thanks