Re: [RFC Part1 PATCH 04/13] x86/sev-snp: define page state change VMGEXIT structure

Brijesh Singh
2021-04-01T17:42:13Z

On 4/1/21 5:32 AM, Borislav Petkov wrote:
> On Wed, Mar 24, 2021 at 11:44:15AM -0500, Brijesh Singh wrote:
>> An SNP-active guest will use the page state change VNAE MGEXIT defined in
> I guess this was supposed to mean "NAE VMGEXIT" but pls write "NAE" out
> at least once so that reader can find its way around the spec.

Noted. I will fix in next rev.

>> the GHCB specification section 4.1.6 to ask the hypervisor to make the
>> guest page private or shared in the RMP table. In addition to the
>> private/shared, the guest can also ask the hypervisor to split or
>> combine multiple 4K validated pages as a single 2M page or vice versa.
>>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Borislav Petkov <bp@alien8.de>
>> Cc: Joerg Roedel <jroedel@suse.de>
>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>> Cc: Tony Luck <tony.luck@intel.com>
>> Cc: Dave Hansen <dave.hansen@intel.com>
>> Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>> Cc: Tom Lendacky <thomas.lendacky@amd.com>
>> Cc: David Rientjes <rientjes@google.com>
>> Cc: Sean Christopherson <seanjc@google.com>
>> Cc: x86@kernel.org
>> Cc: kvm@vger.kernel.org
>> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
>> ---
>> arch/x86/include/asm/sev-snp.h | 34 +++++++++++++++++++++++++++++++++
>> arch/x86/include/uapi/asm/svm.h | 1 +
>> 2 files changed, 35 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/sev-snp.h b/arch/x86/include/asm/sev-snp.h
>> index 5a6d1367cab7..f514dad276f2 100644
>> --- a/arch/x86/include/asm/sev-snp.h
>> +++ b/arch/x86/include/asm/sev-snp.h
>> @@ -22,6 +22,40 @@
>> #define RMP_PG_SIZE_2M 1
>> #define RMP_PG_SIZE_4K 0
>>
>> +/* Page State Change MSR Protocol */
>> +#define GHCB_SNP_PAGE_STATE_CHANGE_REQ 0x0014
>> +#define GHCB_SNP_PAGE_STATE_REQ_GFN(v, o) (GHCB_SNP_PAGE_STATE_CHANGE_REQ | \
>> + ((unsigned long)((o) & 0xf) << 52) | \
>> + (((v) << 12) & 0xffffffffffffff))
> This macro needs to be more readable and I'm not sure the masking is
> correct. IOW, something like this perhaps:
>
> #define GHCB_SNP_PAGE_STATE_REQ_GFN(va, operation) \
> ((((operation) & 0xf) << 52) | ((va) & GENMASK_ULL(51, 12)) | GHCB_SNP_PAGE_STATE_CHANGE_REQ)

I guess I was trying to keep it in consistent with sev-es.h macro
definitions in which the command is used before the fields. In next
version, I will use the msb to lsb ordering.

>
> where you have each GHCBData element at the proper place, msb to lsb.
> Now, GHCB spec says:
>
> "GHCBData[51:12] – Guest physical frame number"
>
> and I'm not clear as to what this macro takes: a virtual address or a
> pfn. If it is a pfn, then you need to do:
>
> (((pfn) << 12) & GENMASK_ULL(51, 0))
>
> but if it is a virtual address you need to do what I have above. Since
> you do "<< 12" then it must be a pfn already but then you should call
> the argument "pfn" so that it is clear what it takes.

Yes, the macro takes the pfn.

> Your mask above covers [55:0] but [55:52] is the page operation so the
> high bit in that mask needs to be 51.

Ack. I will limit the mask to 51 so that we don't go outside the pfn
field width. thank you for pointing it.

> AFAICT, ofc.
>
>> +#define SNP_PAGE_STATE_PRIVATE 1
>> +#define SNP_PAGE_STATE_SHARED 2
>> +#define SNP_PAGE_STATE_PSMASH 3
>> +#define SNP_PAGE_STATE_UNSMASH 4
>> +
>> +#define GHCB_SNP_PAGE_STATE_CHANGE_RESP 0x0015
>> +#define GHCB_SNP_PAGE_STATE_RESP_VAL(val) ((val) >> 32)
> ^^^^^^^^^^^^
>
> Some stray tabs here and above, pls pay attention to vertical alignment too.

I noticed that sev-es.h uses tab when defining the macro to build
command. Another example where I tried to keep a bit consistentency with
sev-es.h. I will drop it in next rev.

>
>> +
>> +/* Page State Change NAE event */
>> +#define SNP_PAGE_STATE_CHANGE_MAX_ENTRY 253
>> +struct __packed snp_page_state_header {
>> + uint16_t cur_entry;
>> + uint16_t end_entry;
>> + uint32_t reserved;
>> +};
>> +
>> +struct __packed snp_page_state_entry {
>> + uint64_t cur_page:12;
>> + uint64_t gfn:40;
>> + uint64_t operation:4;
>> + uint64_t pagesize:1;
>> + uint64_t reserved:7;
>> +};
> Any particular reason for the uint<width>_t types or can you use our
> shorter u<width> types?

IIRC, the spec structure has uint<width>_t, so I used it as-is. No
strong reason for using it. I will switch to u64 type in the next version.

>
>> +
>> +struct __packed snp_page_state_change {
>> + struct snp_page_state_header header;
>> + struct snp_page_state_entry entry[SNP_PAGE_STATE_CHANGE_MAX_ENTRY];
> Also, looking further into the patchset, I wonder if it would make sense
> to do:
>
> s/PAGE_STATE_CHANGE/PSC/g
>
> to avoid breaking lines of huge statements:
>
> + if ((GHCB_SEV_GHCB_RESP_CODE(val) != GHCB_SNP_PAGE_STATE_CHANGE_RESP) ||
> + (GHCB_SNP_PAGE_STATE_RESP_VAL(val) != 0))
> + sev_es_terminate(GHCB_SEV_ES_REASON_GENERAL_REQUEST);
>
> and turn them into something more readable
>
> + if ((GHCB_SEV_GHCB_RESP_CODE(val) != GHCB_SNP_PSC_RESP) ||
> + (GHCB_SNP_PSC_RESP_VAL(val)))
> + sev_es_terminate(GHCB_SEV_ES_REASON_GENERAL_REQUEST);
>
> Also, you have GHCB_SEV and GHCB_SNP prefixes and I wonder whether we
> even need them and have everything be prefixed simply GHCB_ because that
> is the protocol, after all.
>
> Which will then turn the above into:
>
> + if ((GHCB_RESP_CODE(val) != GHCB_PSC_RESP) || (GHCB_PSC_RESP_VAL(val)))
> + sev_es_terminate(GHCB_REASON_GENERAL_REQUEST);
>
> Oh yeah baby! :-)

It looks much better :-). I am good with dropping GHCB_{SEV,SNP} prefix,
and also rename the PAGE_STATE_CHANGE to PSC. Thanks.

>
> Thx.
>