Hi Andrew,
On 20/03/2024 14:39, Andrew Cooper wrote:
On 20/03/2024 2:26 pm, Roger Pau Monné wrote:
On Wed, Mar 20, 2024 at 02:06:27PM +0000, Andrew Cooper wrote:
On 20/03/2024 1:57 pm, Roger Pau Monne wrote:
There's no reason to force HVM guests to have a valid vcpu_info area when
initializing a vCPU, as the vCPU can also be brought online using the local
APIC, and on that path there's no requirement for vcpu_info to be setup ahead
of the bring up. Note an HVM vCPU can operate normally without making use of
vcpu_info.
Restrict the check against dummy_vcpu_info to only apply to PV guests.
Fixes: 192df6f9122d ('x86: allow HVM guests to use hypercalls to bring up
vCPUs')
Signed-off-by: Roger Pau Monné <roger....@citrix.com>
Thanks for looking into this. But, do we actually need to force this on
PV either?
Possibly, having now taken a look at the users it does seem they could
cope with unpopulated vcpu_info_area.
Part of my understanding was that this was some kind of courtesy to PV
guests in order to prevent starting them without a vcpu_info, which
strictly speaking is not mandatory, but otherwise the guest vCPU won't
be able to receive interrupts, not even IPIs.
That's more of a stick than a carrot... "you must set up the area of a
CPU before you can bring it online". Except as we've seen, HVM has been
fine all along without it.
The only interesting user of dummy_vcpu_info now is vcpu_info_populate()
which can cope with any arbitrary vCPU.
I have a suspicion that we can (now) delete these two checks, delete the
dummy_vcpu_info object, and use a regular NULL pointer in
vcpu_info_{populate,reset}(), and in doing so, remove one of the bigger
pieces of PV-absurdity from Xen.
I was expecting this to be something we can backport. OTOH removing
the check completely (or even getting rid of dummy_vcpu_info) is not
something that we would want to backport.
I would rather do the removal of dummy_vcpu_info as followup work.
I was worried about ARM with your patch, because it's spelt HVM there,
rather than PV.
However, I'd forgotten that ARM's do_vcpu_op() filters ops down to just
VCPUOP_register_{vcpu_info,runstate_memory_area} so VCPUOP_initialise
isn't reachable.
Therefore, Reviewed-by: Andrew Cooper <andrew.coop...@citrix.com>
It also means ARM can't use any of the PHYS registration yet...
Whoops. I don't think this was intended. Jan, I don't seem to find any
use in Linux. Do you have any patches you could share? I would like to
give a try on Arm before sending a patch?
Cheers,
--
Julien Grall
