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

[Zeng Guang <guang.zeng@intel.com>]

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.

> 		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@google.com>
> 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@google.com>
> ---
>   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)
> --