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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.02.2020 16:27, Roger Pau Monné wrote:
> On Fri, Feb 28, 2020 at 02:29:09PM +0100, Jan Beulich wrote:
>> 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.
> 
> What about adding:
> 
>>> 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.
> 
> "Most compilers will already do such writes and reads as a single
> instruction, so the usage of atomic operations is mostly used as a
> safety measure."
> 
> Here?

Could you perhaps start with "Compilers will normally ..." I'm fine
with the rest, it's just that "most compilers" still feels like
an understatement.

Jan

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