Re: [PATCH v5 5/8] KVM: x86: Support interrupt dispatch in x2APIC mode with APIC-write VM exit

[Yuan Yao]

On Sat, Jan 15, 2022 at 10:08:10AM +0800, Zeng Guang wrote:
> On 1/15/2022 1:34 AM, Sean Christopherson wrote:
> > On Fri, Jan 14, 2022, Zeng Guang wrote:
> > > kvm_lapic_reg_read() is limited to read up to 4 bytes. It needs extension to
> > > support 64bit read.
> > Ah, right.
> >
> > > And another concern is here getting reg value only specific from vICR(no
> > > other regs need take care), going through whole path on kvm_lapic_reg_read()
> > > could be time-consuming unnecessarily. Is it proper that calling
> > > kvm_lapic_get_reg64() to retrieve vICR value directly?
> > Hmm, no, I don't think that's proper.  Retrieving a 64-bit value really is unique
> > to vICR.  Yes, the code does WARN on that, but if future architectural extensions
> > even generate APIC-write exits on other registers, then using kvm_lapic_get_reg64()
> > would be wrong and this code would need to be updated again.
> Split on x2apic and WARN on (offset != APIC_ICR) already limit register read
> to vICR only. Actually
> we just need consider to deal with 64bit data specific to vICR in APIC-write
> exits. From this point of
> view, previous design can be compatible on handling other registers even if
> future architectural
> extensions changes. :)
> >
> > What about tweaking my prep patch from before to the below?  That would yield:
> >
> > 	if (apic_x2apic_mode(apic)) {
> > 		if (WARN_ON_ONCE(offset != APIC_ICR))
> > 			return 1;
> >
> > 		kvm_lapic_msr_read(apic, offset, &val);
>
> I think it's problematic to use kvm_lapic_msr_read() in this case. It
> premises the high 32bit value
> already valid at APIC_ICR2, while in handling "nodecode" x2APIC writes we
> need get continuous 64bit
> data from offset 300H first and prepare emulation of APIC_ICR2 write. At
> this time, APIC_ICR2 is not
> ready yet.

How about combine them, then you can handle the ICR write vmexit for
IPI virtualization and Sean's patch can still work with code reusing,
like below:

	if (apic_x2apic_mode(apic)) {
		if (WARN_ON_ONCE(offset != APIC_ICR))
			kvm_lapic_msr_read(apic, offset, &val);
		else
			kvm_lapic_get_reg64(apic, offset, &val);

		kvm_lapic_msr_write(apic, offset, val);
	} else {
		kvm_lapic_reg_read(apic, offset, 4, &val);
		kvm_lapic_reg_write(apic, offset, val);
	}

>
> > 		kvm_lapic_msr_write(apic, offset, val);
> > 	} else {
> > 		kvm_lapic_reg_read(apic, offset, 4, &val);
> > 		kvm_lapic_reg_write(apic, offset, val);
> > 	}
> >
> > I like that the above has "msr" in the low level x2apic helpers, and it maximizes
> > code reuse.  Compile tested only...
> >
> > From: Sean Christopherson <seanjc@xxxxxxxxxx>
> > Date: Fri, 14 Jan 2022 09:29:34 -0800
> > Subject: [PATCH] KVM: x86: Add helpers to handle 64-bit APIC MSR read/writes
> >
> > Add helpers to handle 64-bit APIC read/writes via MSRs to deduplicate the
> > x2APIC and Hyper-V code needed to service reads/writes to ICR.  Future
> > support for IPI virtualization will add yet another path where KVM must
> > handle 64-bit APIC MSR reads/write (to ICR).
> >
> > Opportunistically fix the comment in the write path; ICR2 holds the
> > destination (if there's no shorthand), not the vector.
> >
> > No functional change intended.
> >
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---
> >   arch/x86/kvm/lapic.c | 59 ++++++++++++++++++++++----------------------
> >   1 file changed, 29 insertions(+), 30 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index f206fc35deff..cc4531eb448f 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -2787,6 +2787,30 @@ int kvm_lapic_set_vapic_addr(struct kvm_vcpu *vcpu, gpa_t vapic_addr)
> >   	return 0;
> >   }
> >
> > +static int kvm_lapic_msr_read(struct kvm_lapic *apic, u32 reg, u64 *data)
> > +{
> > +	u32 low, high = 0;
> > +
> > +	if (kvm_lapic_reg_read(apic, reg, 4, &low))
> > +		return 1;
> > +
> > +	if (reg == APIC_ICR &&
> > +	    WARN_ON_ONCE(kvm_lapic_reg_read(apic, APIC_ICR2, 4, &high)))
> > +		return 1;
> > +
> > +	*data = (((u64)high) << 32) | low;
> > +
> > +	return 0;
> > +}
> > +
> > +static int kvm_lapic_msr_write(struct kvm_lapic *apic, u32 reg, u64 data)
> > +{
> > +	/* For 64-bit ICR writes, set ICR2 (dest) before ICR (command). */
> > +	if (reg == APIC_ICR)
> > +		kvm_lapic_reg_write(apic, APIC_ICR2, (u32)(data >> 32));
> > +	return kvm_lapic_reg_write(apic, reg, (u32)data);
> > +}
> > +
> >   int kvm_x2apic_msr_write(struct kvm_vcpu *vcpu, u32 msr, u64 data)
> >   {
> >   	struct kvm_lapic *apic = vcpu->arch.apic;
> > @@ -2798,16 +2822,13 @@ int kvm_x2apic_msr_write(struct kvm_vcpu *vcpu, u32 msr, u64 data)
> >   	if (reg == APIC_ICR2)
> >   		return 1;
> >
> > -	/* if this is ICR write vector before command */
> > -	if (reg == APIC_ICR)
> > -		kvm_lapic_reg_write(apic, APIC_ICR2, (u32)(data >> 32));
> > -	return kvm_lapic_reg_write(apic, reg, (u32)data);
> > +	return kvm_lapic_msr_write(apic, reg, data);
> >   }
> >
> >   int kvm_x2apic_msr_read(struct kvm_vcpu *vcpu, u32 msr, u64 *data)
> >   {
> >   	struct kvm_lapic *apic = vcpu->arch.apic;
> > -	u32 reg = (msr - APIC_BASE_MSR) << 4, low, high = 0;
> > +	u32 reg = (msr - APIC_BASE_MSR) << 4;
> >
> >   	if (!lapic_in_kernel(vcpu) || !apic_x2apic_mode(apic))
> >   		return 1;
> > @@ -2815,45 +2836,23 @@ int kvm_x2apic_msr_read(struct kvm_vcpu *vcpu, u32 msr, u64 *data)
> >   	if (reg == APIC_DFR || reg == APIC_ICR2)
> >   		return 1;
> >
> > -	if (kvm_lapic_reg_read(apic, reg, 4, &low))
> > -		return 1;
> > -	if (reg == APIC_ICR)
> > -		kvm_lapic_reg_read(apic, APIC_ICR2, 4, &high);
> > -
> > -	*data = (((u64)high) << 32) | low;
> > -
> > -	return 0;
> > +	return kvm_lapic_msr_read(apic, reg, data);
> >   }
> >
> >   int kvm_hv_vapic_msr_write(struct kvm_vcpu *vcpu, u32 reg, u64 data)
> >   {
> > -	struct kvm_lapic *apic = vcpu->arch.apic;
> > -
> >   	if (!lapic_in_kernel(vcpu))
> >   		return 1;
> >
> > -	/* if this is ICR write vector before command */
> > -	if (reg == APIC_ICR)
> > -		kvm_lapic_reg_write(apic, APIC_ICR2, (u32)(data >> 32));
> > -	return kvm_lapic_reg_write(apic, reg, (u32)data);
> > +	return kvm_lapic_msr_write(vcpu->arch.apic, reg, data);
> >   }
> >
> >   int kvm_hv_vapic_msr_read(struct kvm_vcpu *vcpu, u32 reg, u64 *data)
> >   {
> > -	struct kvm_lapic *apic = vcpu->arch.apic;
> > -	u32 low, high = 0;
> > -
> >   	if (!lapic_in_kernel(vcpu))
> >   		return 1;
> >
> > -	if (kvm_lapic_reg_read(apic, reg, 4, &low))
> > -		return 1;
> > -	if (reg == APIC_ICR)
> > -		kvm_lapic_reg_read(apic, APIC_ICR2, 4, &high);
> > -
> > -	*data = (((u64)high) << 32) | low;
> > -
> > -	return 0;
> > +	return kvm_lapic_msr_read(vcpu->arch.apic, reg, data);
> >   }
> >
> >   int kvm_lapic_set_pv_eoi(struct kvm_vcpu *vcpu, u64 data, unsigned long len)
> > --