Re: [PATCH] x86/hvm: Allow supplying a dynamic start ASID

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 16/04/2024 9:54 am, Vaishali Thakkar wrote:
> Currently, Xen always starts the ASID allocation at 1. But
> for SEV technologies the ASID space is divided. This is
> because it's a security issue if a guest is started as
> ES/SNP and is migrated to SEV-only. So, the types are
> tracked explicitly.
>
> Thus, in preparation of SEV support in Xen, add min_asid
> to allow supplying the dynamic start ASID during the
> allocation process.
>
> Signed-off-by: Vaishali Thakkar <vaishali.thakkar@vates.tech>

Mechanically, this is fine, but is it going to be useful in the long term?

For SEV and SEV-ES/SNP, we must run the VM in the single fixed ASID
negotiated with the ASP.  This is not not optional.

For non-encrypted VMs, we are free to choose between using the remaining
ASID space as we used to (i.e. run it per-pCPU and tick it over to flush
the TLBs), or to run it in a fixed ASID per guest too.

The latter lets us use INVLPGB, and would avoid maintaining two
different TLB-tagging schemes.


I'd say that we absolutely do want INVLPGB support for managing
non-encrypted VMs, and we cannot mix both fixed and floating ASIDs at
the same time.

We should seriously consider whether it's worth maintaining two schemes,
or just switching wholesale to a fixed ASID scheme.

I don't have a good answer here...  If it where anything but TLB
flushing, it would be an obvious choice, but TLB handling bugs are some
of the nastiest to show up.

~Andrew