Re: [PATCH v7 5/8] KVM: x86: Add support for vICR APIC-write VM-Exits in x2APIC mode

[Sean Christopherson <seanjc@google.com>]

On Fri, Mar 04, 2022, Zeng Guang wrote:
> Upcoming Intel CPUs will support virtual x2APIC MSR writes to the vICR,
> i.e. will trap and generate an APIC-write VM-Exit instead of intercepting
> the WRMSR.  Add support for handling "nodecode" x2APIC writes, which
> were previously impossible.
> 
> Note, x2APIC MSR writes are 64 bits wide.
> 
> Signed-off-by: Zeng Guang <guang.zeng@intel.com>
> ---
>  arch/x86/kvm/lapic.c | 22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 629c116b0d3e..22929b5b3f9b 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -67,6 +67,7 @@ static bool lapic_timer_advance_dynamic __read_mostly;
>  #define LAPIC_TIMER_ADVANCE_NS_MAX     5000
>  /* step-by-step approximation to mitigate fluctuation */
>  #define LAPIC_TIMER_ADVANCE_ADJUST_STEP 8
> +static int kvm_lapic_msr_read(struct kvm_lapic *apic, u32 reg, u64 *data);
>  
>  static inline void __kvm_lapic_set_reg(char *regs, int reg_off, u32 val)
>  {
> @@ -2227,10 +2228,25 @@ EXPORT_SYMBOL_GPL(kvm_lapic_set_eoi);
>  /* emulate APIC access in a trap manner */
>  void kvm_apic_write_nodecode(struct kvm_vcpu *vcpu, u32 offset)
>  {
> -	u32 val = kvm_lapic_get_reg(vcpu->arch.apic, offset);
> +	struct kvm_lapic *apic = vcpu->arch.apic;
> +	u64 val;
> +
> +	if (apic_x2apic_mode(apic)) {
> +		/*
> +		 * When guest APIC is in x2APIC mode and IPI virtualization
> +		 * is enabled, accessing APIC_ICR may cause trap-like VM-exit
> +		 * on Intel hardware. Other offsets are not possible.
> +		 */
> +		if (WARN_ON_ONCE(offset != APIC_ICR))
> +			return;
>  
> -	/* TODO: optimize to just emulate side effect w/o one more write */
> -	kvm_lapic_reg_write(vcpu->arch.apic, offset, val);
> +		kvm_lapic_msr_read(apic, offset, &val);
> +		kvm_apic_send_ipi(apic, (u32)val, (u32)(val >> 32));

This needs to clear the APIC_ICR_BUSY bit.  It'd also be nice to trace this write.
The easiest thing is to use kvm_x2apic_icr_write().  Kinda silly as it'll generate
an extra write, but on the plus side the TODO comment doesn't have to move :-D

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index c4c3155d98db..58bf296ee313 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2230,6 +2230,7 @@ void kvm_apic_write_nodecode(struct kvm_vcpu *vcpu, u32 offset)
        struct kvm_lapic *apic = vcpu->arch.apic;
        u64 val;

+       /* TODO: optimize to just emulate side effect w/o one more write */
        if (apic_x2apic_mode(apic)) {
                /*
                 * When guest APIC is in x2APIC mode and IPI virtualization
@@ -2240,10 +2241,9 @@ void kvm_apic_write_nodecode(struct kvm_vcpu *vcpu, u32 offset)
                        return;

                kvm_lapic_msr_read(apic, offset, &val);
-               kvm_apic_send_ipi(apic, (u32)val, (u32)(val >> 32));
+               kvm_x2apic_icr_write(apic, val);
        } else {
                val = kvm_lapic_get_reg(apic, offset);
-               /* TODO: optimize to just emulate side effect w/o one more write */
                kvm_lapic_reg_write(apic, offset, (u32)val);
        }
 }


> +	} else {
> +		val = kvm_lapic_get_reg(apic, offset);
> +		/* TODO: optimize to just emulate side effect w/o one more write */
> +		kvm_lapic_reg_write(apic, offset, (u32)val);
> +	}
>  }
>  EXPORT_SYMBOL_GPL(kvm_apic_write_nodecode);
>  
> -- 
> 2.27.0
>