Re: [RFC v2] KVM: arm/arm64: optimize vSGI injection performance

[zhaoxu]

On 2023/9/4 17:57, Marc Zyngier wrote:
> On Fri, 25 Aug 2023 02:58:11 +0100,
> Xu Zhao <zhaoxu.35@xxxxxxxxxxxxx> wrote:
[...]
> > -	unsigned long affinity;
> > -	int level0;
> > +	u64 aff;
> >   
> > -	/*
> > -	 * Split the current VCPU's MPIDR into affinity level 0 and the
> > -	 * rest as this is what we have to compare against.
> > -	 */
> > -	affinity = kvm_vcpu_get_mpidr_aff(vcpu);
> > -	level0 = MPIDR_AFFINITY_LEVEL(affinity, 0);
> > -	affinity &= ~MPIDR_LEVEL_MASK;
> > +	/* aff3 - aff1 */
> > +	aff = (((reg) & ICC_SGI1R_AFFINITY_3_MASK) >> ICC_SGI1R_AFFINITY_3_SHIFT) << 16 |
> > +		(((reg) & ICC_SGI1R_AFFINITY_2_MASK) >> ICC_SGI1R_AFFINITY_2_SHIFT) << 8 |
> > +		(((reg) & ICC_SGI1R_AFFINITY_1_MASK) >> ICC_SGI1R_AFFINITY_1_SHIFT);
>
> Here, you assume that you can directly map a vcpu index to an
> affinity. It would be awesome if that was the case. However, this is
> only valid at reset time, and userspace is perfectly allowed to change
> this mapping by writing to the vcpu's MPIDR_EL1.
>
> So this won't work at all if userspace wants to set its own specific
> CPU numbering.
>
> 	M.
Hi Marc,

Yes, i don't think too much about userspace can change MPIDR value, I 
checked the source code of qemu, qemu create vcpu sequentially, so in 
this case, vcpu_id is equivalent to vcpu_idx which means vcpu_id 
represents the position in vcpu array.

These days, I'm still thinking about whether it is because of the 
content related to future vcpu hot-plug feature that vcpu_id can be 
modified, but now it seems that's not entirely the case.

I have read your latest patch and have been deeply inspired, and Thanks 
for agreeing with this issue.

With Regards
	Xu.