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Re: [PATCH] sev: allow capabilities to check for SEV-ES support

From: Tyler Fanelli
Subject: Re: [PATCH] sev: allow capabilities to check for SEV-ES support
Date: Tue, 16 Nov 2021 11:58:12 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0

On 11/16/21 10:53 AM, Daniel P. Berrangé wrote:
On Tue, Nov 16, 2021 at 10:29:35AM -0500, Tyler Fanelli wrote:
On 11/16/21 4:17 AM, Daniel P. Berrangé wrote:
On Mon, Nov 15, 2021 at 02:38:04PM -0500, Tyler Fanelli wrote:
Probe for SEV-ES and SEV-SNP capabilities to distinguish between Rome,
Naples, and Milan processors. Use the CPUID function to probe if a
processor is capable of running SEV-ES or SEV-SNP, rather than if it
actually is running SEV-ES or SEV-SNP.

Signed-off-by: Tyler Fanelli <tfanelli@redhat.com>
   qapi/misc-target.json | 11 +++++++++--
   target/i386/sev.c     |  6 ++++--
   2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/qapi/misc-target.json b/qapi/misc-target.json
index 5aa2b95b7d..c3e9bce12b 100644
--- a/qapi/misc-target.json
+++ b/qapi/misc-target.json
@@ -182,13 +182,19 @@
   # @reduced-phys-bits: Number of physical Address bit reduction when SEV is
   #                     enabled
+# @es: SEV-ES capability of the machine.
+# @snp: SEV-SNP capability of the machine.
   # Since: 2.12
   { 'struct': 'SevCapability',
     'data': { 'pdh': 'str',
               'cert-chain': 'str',
               'cbitpos': 'int',
-            'reduced-phys-bits': 'int'},
+            'reduced-phys-bits': 'int',
+            'es': 'bool',
+            'snp': 'bool'},
     'if': 'TARGET_I386' }
@@ -205,7 +211,8 @@
   # -> { "execute": "query-sev-capabilities" }
   # <- { "return": { "pdh": "8CCDD8DDD", "cert-chain": "888CCCDDDEE",
-#                  "cbitpos": 47, "reduced-phys-bits": 5}}
+#                  "cbitpos": 47, "reduced-phys-bits": 5
+#                  "es": false, "snp": false}}
We've previously had patches posted to support SNP in QEMU


and this included an update to query-sev for reporting info
about the VM instance.

Your patch is updating query-sev-capabilities, which is a
counterpart for detecting host capabilities separate from
a guest instance.
Yes, that's because with this patch, I'm more interested in determining
which AMD processor is running on a host, and less if ES or SNP is actually
running on a guest instance or not.
None the less I wonder if the same design questions from
query-sev apply. ie do we need to have the ability to
report any SNP specific information fields, if so we need
to use a discriminated union of structs, not just bool

More generally I'm some what wary of adding this to
query-sev-capabilities at all, unless it is part of the
main SEV-SNP series.

Also what's the intended usage for the mgmt app from just
having these boolean fields ? Are they other more explicit
feature flags we should be reporting, instead of what are
essentially SEV generation codenames.
If by "mgmt app" you're referring to sevctl, in order to determine which
certificate chain to use (Naples vs Rome vs Milan ARK/ASK) we must query
which processor we are running on. Although sevctl has a feature which can
do this already, we cannot guarantee that sevctl is running on the same host
that a VM is running on, so we must query this capability from QEMU. My
logic was determining the processor would have been the following:
I'm not really talking about a specific, rather any tool which wants
to deal with SEV and QEMU, whether libvirt or an app using libvirt,
or something else using QEMU directly.

Ah, my mistake.

Where does the actual cert chain payload come from ? Is that something
the app has to acquire out of band, or can the full cert chain be
acquired from the hardware itself ?

The cert chain (or the ARK/ASK specifically) comes from AMD's KDS, yet sevctl is able to cache the values, and has them on-hand when needed. This patch would tell sevctl *which* of the cert chains to use (Naples vs Rome vs Milan chain). If need be, I could just focus on Naples and Rome processors for now and bring support for SNP (Milan processors) later on when it is more mature.

!es && !snp --> Naples

es && !snp --> Rome

es && snp --> Milan
This approach isn't future proof if subsequent generations introduce
new certs. It feels like we should be explicitly reporting something
about the certs rather than relying on every app to re-implement tihs

Alright, like an encoding of which processor generation the host is running on?



Tyler Fanelli (tfanelli)

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