Re: [Xen-devel] [PATCH v5 1/7] x86/hvm: allow ASID flush when v != current

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.02.2020 18:43, Roger Pau Monne wrote:
> Current implementation of hvm_asid_flush_vcpu is not safe to use
> unless the target vCPU is either paused or the currently running one,
> as it modifies the generation without any locking.

Indeed, but the issue you're taking care of is highly theoretical:
I don't think any sane compiler will split writes of the fields
to multiple insns. It would be nice if this was made clear here.

> Fix this by using atomic operations when accessing the generation
> field, both in hvm_asid_flush_vcpu_asid and other ASID functions. This
> allows to safely flush the current ASID generation. Note that for the
> flush to take effect if the vCPU is currently running a vmexit is
> required.
> 
> Note the same could be achieved by introducing an extra field to
> hvm_vcpu_asid that signals hvm_asid_handle_vmenter the need to call
> hvm_asid_flush_vcpu on the given vCPU before vmentry, this however
> seems unnecessary as hvm_asid_flush_vcpu itself only sets two vCPU
> fields to 0, so there's no need to delay this to the vmentry ASID
> helper.
> 
> This is not a bugfix as no callers that would violate the assumptions
> listed in the first paragraph have been found, but a preparatory
> change in order to allow remote flushing of HVM vCPUs.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Reviewed-by: Wei Liu <wl@xxxxxxx>

With a suitable clarification added
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan

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