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Re: [PATCH] x86/vcpu: relax VCPUOP_initialise restriction for non-PV vCPUs
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 20 Mar 2024 16:03:25 +0100
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- Cc: George Dunlap <george.dunlap@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 20 Mar 2024 15:03:39 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 20.03.2024 15:06, 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.pau@xxxxxxxxxx>
>
> Thanks for looking into this. But, do we actually need to force this on
> PV either?
>
> The only interesting user of dummy_vcpu_info now is vcpu_info_populate()
> which can cope with any arbitrary vCPU.
Why would that be the only interesting user? If we were to ...
> 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.
... use NULL pointers instead, we'd need to audit all Xen's vcpu_info
accesses for properly avoiding to de-reference that NULL. The dummy
struct instance was really meant as a safety belt (besides also acting
as a sentinel).
> I can entirely believe that this is only safe to do with Jan's physical
> registration changes in place.
Which nobody uses just yet, afaik.
Jan
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