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

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Message-ID: <28610076-6ac7-4004-8479-e0005ff96c63@bytedance.com>
Date:   Tue, 12 Sep 2023 12:13:19 +0800
From:   zhaoxu <zhaoxu.35@...edance.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     oliver.upton@...ux.dev, james.morse@....com,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        zhouyibo@...edance.com, zhouliang.001@...edance.com
Subject: Re: [RFC v2] KVM: arm/arm64: optimize vSGI injection performance



On 2023/9/4 17:57, Marc Zyngier wrote:
> On Fri, 25 Aug 2023 02:58:11 +0100,
> Xu Zhao <zhaoxu.35@...edance.com> 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.