Re: Live migration regarding Intel PT

[Eduardo Habkost]

On Wed, Aug 25, 2021 at 02:59:37PM +0800, Xiaoyao Li wrote:
> Hi Eduardo,
> 
> I have some question regrading Intel PT live migration.
> 
> Commit "e37a5c7fa459 (i386: Add Intel Processor Trace feature support)"
> expose Intel PT with a fixed capabilities of CPUID 0x14 for live migration.
> And the fixed capabilities are the value reported on ICX(IceLake). However,
> the upcoming SPR(Sapphire Rapids) has less capabilities of
> INTEL_PT_CYCLE_BITMAP than ICX. It fails to enable PT in guest on SPR
> machine.
> 
> If change the check on INTEL_PT_CYCLE_BITMAP to allow different value to
> allow it work on SPR. I think it breaks live migration, right?

If I understand your description correctly, I don't think that
would break live migration.

Generating different CPUID data for the same CPU model+flags
would break live migration.

However, merely allowing a wider set of configurations to work
wouldn't break live migration, as long as the same CPU
model+flags generates the same CPUID data on any host.


> 
> For me, not making each sub-function of PT as configurable in QEMU indeed
> makes it hard for live migration. [...]

Making all sub-functions of PT configurable would be welcome.
actually.  That would allow us to support a wider range of
configurations and keep live migration working at the same time.


> [...] Why not make PT as unmigratable in the
> first place when introducing the support in QEMU?

I don't know, this was not my decision.  Now we support live
migration in a few specific cases (ICX hosts), and I assume we
don't want to stop supporting it.

If you just want to support a larger set of hosts when live
migration is not needed, you can add support to that use case
too.  e.g.: "-cpu host,migratable=off" and/or
"-cpu ...,intel-pt-passthrough=on" could do host passthrough of
intel-pt sub-leaves (and block live migration).

-- 
Eduardo