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Date:   Fri, 17 Dec 2021 16:19:25 -0600
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Subject: Re: [PATCH v8 08/40] x86/sev: Check the vmpl level
Content-Language: en-US
To:     Mikolaj Lisik <lisik@google.com>,
        Venu Busireddy <venu.busireddy@oracle.com>
References: <20211210154332.11526-1-brijesh.singh@amd.com>
 <20211210154332.11526-9-brijesh.singh@amd.com> <YbugbgXhApv9ECM2@dt>
 <CADtC8PX_bEk3rQR1sonbp-rX7rAG4fdbM41r3YLhfj3qWvqJrw@mail.gmail.com>
From:   Brijesh Singh <brijesh.singh@amd.com>
In-Reply-To: <CADtC8PX_bEk3rQR1sonbp-rX7rAG4fdbM41r3YLhfj3qWvqJrw@mail.gmail.com>
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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