Re: [PATCH v6 5/9] KVM: x86: Add support for vICR APIC-write VM-Exits in x2APIC mode

[Chao Gao <chao.gao@intel.com>]

On Fri, Feb 25, 2022 at 04:44:05PM +0200, Maxim Levitsky wrote:
>On Fri, 2022-02-25 at 16:22 +0800, 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 | 25 ++++++++++++++++++++++---
>>  1 file changed, 22 insertions(+), 3 deletions(-)
>> 
>> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
>> index 629c116b0d3e..e4bcdab1fac0 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 +2228,28 @@ 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);
>> +		if (val & APIC_ICR_BUSY)
>> +			kvm_x2apic_icr_write(apic, val);
>> +		else
>> +			kvm_apic_send_ipi(apic, (u32)val, (u32)(val >> 32));
>I don't fully understand the above code.
>
>First of where kvm_x2apic_icr_write is defined?

Sean introduces it in his "prep work for VMX IPI virtualization" series, which
is merged into kvm/queue branch.

https://git.kernel.org/pub/scm/virt/kvm/kvm.git/commit/?h=queue&id=7a641ca0c219e4bbe102f2634dbc7e06072fcd3c

>
>Second, I thought that busy bit is not used in x2apic mode?
>At least in intel's SDM, section 10.12.9 'ICR Operation in x2APIC Mode'
>this bit is not defined.

You are right. We will remove the pointless check against APIC_ICR_BUSY and
just invoke kvm_apic_send_ipi().

In that section, SDM also says:
With the removal of the Delivery Status bit, system software no longer has a
reason to read the ICR. It remains readable only to aid in debugging; however,
***software should not assume the value returned by reading the ICR is the last
written value***.