RE: [v3 23/26] KVM: Update Posted-Interrupts Descriptor when vCPU is preempted

[Wu, Feng]

> -----Original Message-----
> From: Paolo Bonzini [mailto:paolo.bonzini@xxxxxxxxx] On Behalf Of Paolo
> Bonzini
> Sent: Thursday, December 18, 2014 1:11 AM
> To: Wu, Feng; Thomas Gleixner; Ingo Molnar; H. Peter Anvin; x86@xxxxxxxxxx;
> Gleb Natapov; Paolo Bonzini; dwmw2@xxxxxxxxxxxxx; joro@xxxxxxxxxx; Alex
> Williamson; jiang.liu-VuQAYsv1563Yd54FQh9/CA@xxxxxxxxxxxxxxxx
> Cc: iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; KVM list;
> Eric Auger
> Subject: Re: [v3 23/26] KVM: Update Posted-Interrupts Descriptor when vCPU
> is preempted
> 
> 
> 
> On 12/12/2014 16:14, Feng Wu wrote:
> > +	if (irq_remapping_cap(IRQ_POSTING_CAP)) {
> > +		struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
> > +		struct pi_desc old, new;
> > +		unsigned int dest;
> > +
> > +		memset(&old, 0, sizeof(old));
> > +		memset(&new, 0, sizeof(new));
> 
> This is quite expensive.  Just use an u64 for old_control and
> new_control, instead of a full struct.
> 
> >
> > +			pi_clear_sn(&new);
> 
> This can be simply new.sn = 0.  It does not need atomic operations.

Thanks for your comments, Paolo!

If we use u64 new_control, we cannot use new.sn any more.
Maybe we can change the struct pi_desc {} like this:

typedef struct pid_control{
        u64     on      : 1,
        sn      : 1,
        rsvd_1  : 13,
        ndm     : 1,
        nv      : 8,
        rsvd_2  : 8,
        ndst    : 32;
}pid_control_t;

struct pi_desc {
        u32 pir[8];     /* Posted interrupt requested */
		pid_control_t control;
        u32 rsvd[6];
} __aligned(64);


Then we can define pid_control_t new_control, old_control. And use new_control.sn = 0.

What is your opinon?

Thanks,
Feng

> 
> Same in patch 24 (if needed at all there---see the reply there).
> 
> >
> > +	if (irq_remapping_cap(IRQ_POSTING_CAP)) {
> > +		struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
> > +		struct pi_desc old, new;
> > +
> > +		memset(&old, 0, sizeof(old));
> > +		memset(&new, 0, sizeof(new));
> > +
> 
> Here you do not need old/new at all because...
> 
> > +		if (vcpu->preempted) {
> > +			do {
> > +				old.control = new.control = pi_desc->control;
> > +				pi_set_sn(&new);
> > +			} while (cmpxchg(&pi_desc->control, old.control,
> > +					new.control) != old.control);
> 
> this can do pi_set_sn directly on pi_desc, without the cmpxchg.
> 
> Paolo
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