Turns out the limit can be obtained from CPUID results so doesn't need kernel support:

  "We can query the limit through the CPUID Fn0x8000_001f[ECX]."

https://www.redhat.com/archives/libvir-list/2019-March/msg01086.html

(In reply to Daniel Berrange from comment #1)
> Turns out the limit can be obtained from CPUID results so doesn't need
> kernel support:
>
>   "We can query the limit through the CPUID Fn0x8000_001f[ECX]."
>
> https://www.redhat.com/archives/libvir-list/2019-March/msg01086.html

sev_get_capabilities() already does

    host_cpuid(0x8000001F, 0, NULL, &ebx, NULL, NULL);
    cap->cbitpos = ebx & 0x3f;

for determining the "C-bit location in page table entry" (see
@SevCapability in "qapi/target.json").

Hopefully it's a "straightforward" [*] addition to the above
host_cpuid() function call -- retrieve ECX too, and propagate the
information in a new field of @SevCapability.

[*] famous last words

--*--

... According to "devel/qapi-code-gen.txt", section "Struct types":

> On output structures (only mentioned in the 'returns' side of a
> command), [...] [c]hanging from optional to mandatory is safe.

and introducing a new (optional or mandatory) field in an output-only
struct is a special case of the above (it's like an optional field that
was never actually produced before that we're now making mandatory).

--*--

Brijesh: is ECX *guaranteed* to contain the needed information, even
with the earliest shipped version of SEV?

... AMD publication #55766 (rev 3.07) "Secure Encrypted Virtualization
API Version 0.17" <https://developer.amd.com/sev/> states,

> 6.17 ACTIVATE
> 6.17.1 Actions
>
> [...]
>
> If the guest is SEV-ES enabled, then the ASID must be at least 1h and
> at most (MIN_SEV_ASID- 1). If the guest is not SEV-ES enabled, then
> the ASID must be at least MIN_SEV_ASID and at most the maximum SEV
> ASID available. The MIN_SEV_ASID value is discovered by CPUID
> Fn8000_001F[EDX]. The maximum SEV ASID available is discovered by
> CPUID Fn8000_001F[ECX].

Based on this, it looks like we can get the simultaneous max guest
*count* from (ECX-EDX+1) -- however, that only applies to non-SEV-ES
guests.

I wonder if both the SEV-ES and non-SEV-ES guest counts should be
exposed at once in the new @SevCapability fields. What's the right level
of abstraction here? Let QEMU compute the differences and only give
those differences (i.e. counts) to libvirtd, or let QEMU expose the
actual ASID limits to libvirtd, and let libvirtd calculate the counts?

Based on the other fields in @SevCapability, I'd say we're already
exposing pretty low-level / raw details (such as "cbitpos"), so maybe we
should propagate MIN_SEV_ASID and MAX_SEV_ASID to libvirtd transparently
too.

Fn8000_001F[ECX] will contain the maximum number of ASID's in current and future hardware. So it's safe to use this in kernel and Qemu to calculate the maximum number of SEV guest. I am thinking in SEV-ES patches we will extend the qemu capabilities to expose the following new fields:

SEV Capabilities {
...
...  
  sev-es = true (bool)
  sev-es-max-guest = n (maximum num of simultaneous SEV-ES guest) [EDX]
  sev-max-guest = n    (maxium number of simultaneous SEV Guest) [ECX-sev-es-max-guest+1]
...
...
}

Hi Brijesh,

(In reply to BRIJESH SINGH from comment #3)

> I am thinking in SEV-ES patches we will
> extend the qemu capabilities to expose the following new fields:
> 
> SEV Capabilities {
> ...
> ...  
>   sev-es = true (bool)
>   sev-es-max-guest = n (maximum num of simultaneous SEV-ES guest) [EDX]
>   sev-max-guest = n    (maxium number of simultaneous SEV Guest)
> [ECX-sev-es-max-guest+1]
> ...
> ...
> }

thanks for the info. Does that mean you are already tracking this feature (including the "sev-max-guest" field) in some upstream tracker item (or patch set even)?

Because then I could make this RHBZ dependent on that upstream tracker, and the RHBZ scope would be reduced to backporting AMD's upstream patches. Thanks.

Hi Laszlo,

I don't have BZ for this. Sometime back we had a discussion about this on libvirt ML and I created TODO task in our internal tracker. If you want then I can submit sev-max-guest field patch in QEMU. I am okay if you submit the patch and copy me for Ack. Let me know whatever works for you. The other fields can be added when we submit the SEV-ES patches.

thanks

Hi Brijesh,

my plate is pretty full, and due to this change modifying a QAPI schema, I expect the upstream patch to go up to v2 at the least, with the review round-trip time that's usual for QEMU. If you can squeeze the upstream patch into your workload, so that I only have backport the upstream commit to RHEL8, I'd prefer that.

Thanks!
Laszlo

Hi Laszlo,

I will submit the patch this week (probably tomorrow) and copy you.

thanks

v1 posted https://patchwork.kernel.org/patch/10896557/

v2: http://mid.mail-archive.com/20190411235456.12918-1-brijesh.singh@amd.com

(In reply to Laszlo Ersek from comment #9)
> v2: http://mid.mail-archive.com/20190411235456.12918-1-brijesh.singh@amd.com

This QEMU patch was straight-forward, and got quickly approved by multiple reviewers; however, Paolo preferred libvirtd to incorporate the check directly.

I'm moving this BZ to libvirtd then.

*** Bug 1689195 has been marked as a duplicate of this bug. ***

Note that the 2nd gen and onwards AMD hardware has much higher limits, not the 15 on the 1st gen; so this is less of an issue.
(And on RHEL we don't support 1st gen for SEV).
Note however that there tend to be BIOS settings to set the number of SEV ASIDs available, so you can run into the limits
due to config.

Bulk update: Move RHEL-AV bugs to RHEL9. If necessary to resolve in RHEL8, then clone to the current RHEL8 release.

After evaluating this issue, there are no plans to address it further or fix it in an upcoming release.  Therefore, it is being closed.  If plans change such that this issue will be fixed in an upcoming release, then the bug can be reopened.

Tyler - sending this your way as something to implement within libvirt. I figure this would be something libvirt should display via the virsh domcapabilities output. I'm sure Daniel can elaborate more.

Note that this is already implemented in libvirt upstream:

commit 7826148a72c97367fc6aaa76397fe92d32169723
Author: Daniel P. Berrangé <berrange>
Date:   Wed Dec 8 14:53:45 2021 -0500

    qemu: report max number of SEV guests
    
    Different CPU generations have different limits on the number
    of SEV/SEV-ES guests that can be run. Since both limits come
    from the same overall set, there is typically also BIOS config
    to set the tradeoff betweeen SEV and SEV-ES guest limits.
    
    This is important information to expose for a mgmt application
    scheduling guests to hosts.
    
    Reviewed-by: Peter Krempa <pkrempa>
    Signed-off-by: Daniel P. Berrangé <berrange>

Thanks Peter - I'll reassign to Daniel, add this to 8.6.0 w/ devel_ack+ and let Daniel do the libvirt & POST magic. I assume it'll be libvirt-8.0.0

Verify this bug with libvirt-daemon-8.0.0-1.module+el8.6.0+13888+55157bfb.x86_64:

1. prepare a EPYC-Milan system and enable SEV and SEV-ES

# lscpu
...
BIOS Vendor ID:      AMD
CPU family:          25
Model:               1
...

# dmesg |grep SEV
[ 1896.000535] SEV supported: 508 ASIDs
[ 1896.004122] SEV-ES supported: 1 ASIDs

2. check virsh domcapabilities sev support status:

# virsh domcapabilities
...
    <sev supported='yes'>
      <cbitpos>51</cbitpos>
      <reducedPhysBits>1</reducedPhysBits>
      <maxGuests>508</maxGuests>
      <maxESGuests>1</maxESGuests>
    </sev>
...

3. try to start 2 (> maxESGuests) sev-es guests and get error:

# virsh start vm2
error: Failed to start domain 'vm2'
error: internal error: process exited while connecting to monitor: 2022-01-21T06:56:49.488273Z qemu-kvm: -accel kvm: sev_kvm_init: failed to initialize ret=-16 fw_error=0 ''
2022-01-21T06:56:49.488454Z qemu-kvm: -accel kvm: failed to initialize kvm: Operation not permitted


And test on a EPYC host which not support sev-es

# virsh domcapabilities
...
    <sev supported='yes'>
      <cbitpos>47</cbitpos>
      <reducedPhysBits>1</reducedPhysBits>
      <maxGuests>15</maxGuests>
      <maxESGuests>0</maxESGuests>
    </sev>
...

And test on a host which not enable sev

# virsh domcapabilities
...
    <sev supported='no'/>
...

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (Moderate: virt:rhel and virt-devel:rhel security, bug fix, and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2022:1759