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 1/18/2022 8:44 AM, Yuan Yao wrote:
> 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);
> 	}

Alternatively we can merge as this if Sean think it ok to call 
kvm_lapic_get_reg64() retrieving 64bit data from vICR directly.

>>> 		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)
>>> --