linux-kernel - Re: [PART1 RFC v2 07/10] svm: Add VMEXIT handlers for AVIC

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Date:	Wed, 9 Mar 2016 21:55:13 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
Cc:	pbonzini@...hat.com, joro@...tes.org, bp@...en8.de,
	gleb@...nel.org, alex.williamson@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, wei@...hat.com,
	sherry.hurwitz@....com
Subject: Re: [PART1 RFC v2 07/10] svm: Add VMEXIT handlers for AVIC

2016-03-04 14:46-0600, Suravee Suthikulpanit:
> From: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
> 
> Introduce VMEXIT handlers, avic_incp_ipi_interception() and
> avic_noaccel_interception().
> 
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
> ---
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> @@ -3690,6 +3690,264 @@ static int mwait_interception(struct vcpu_svm *svm)
> +	case AVIC_INCMP_IPI_ERR_TARGET_NOT_RUN: {
> +		kvm_for_each_vcpu(i, vcpu, kvm) {
> +			if (!kvm_apic_match_dest(vcpu, apic,
> +						 icrl & APIC_SHORT_MASK,
> +						 GET_APIC_DEST_FIELD(icrh),
> +						 icrl & APIC_DEST_MASK))
> +				continue;
> +
> +			kvm_vcpu_kick(vcpu);

KVM shouldn't kick VCPUs that are running.  (Imagine a broadcast when
most VCPUs are in guest mode.)

I think a new helper might be useful here: we only want to wake up from
wait queue, but never force VCPU out of guest mode ... kvm_vcpu_kick()
does both.

> +static int avic_noaccel_trap_write(struct vcpu_svm *svm)
> +{
> +	switch (offset) {
> +	case APIC_ID: {
> +	case APIC_LDR: {
> +	case APIC_DFR: {
> +	}

It's not enough to modify the AVIC map here.  Userspace can also change
the APIC page with kvm_vcpu_ioctl_set_lapic, so AVIC would better hook
into some common path.

I think that AVIC map should be connected to recalculate_apic_map() and
'struct kvm_apic_map' as we already have the mode and a coupling of
LAPICs and VCPUs there.

recalculate_apic_map() is currently quite wasteful as it recomputes the
whole map on every change, but its simplicity should be bearable.

> +static int avic_noaccel_interception(struct vcpu_svm *svm)
> +{
> +	int ret = 0;
> +	u32 offset = svm->vmcb->control.exit_info_1 & 0xFF0;
> +	u32 rw = (svm->vmcb->control.exit_info_1 >> 32) & 0x1;

Change "u32 rw" to "bool write"

> +	u32 vector = svm->vmcb->control.exit_info_2 & 0xFFFFFFFF;

and please #define those masks.

> +	pr_debug("%s: offset=%#x, rw=%#x, vector=%#x, vcpu_id=%#x, cpu=%#x\n",
> +		 __func__, offset, rw, vector, svm->vcpu.vcpu_id, svm->vcpu.cpu);
> +
> +	BUG_ON(offset >= 0x400);

These are valid faulting registers, so our implementation has to handle
them.  (And the rule is to never BUG if a recovery is simple.)

> +	switch (offset) {
> +	case APIC_ID:
> +	case APIC_EOI:
> +	case APIC_RRR:
> +	case APIC_LDR:
> +	case APIC_DFR:
> +	case APIC_SPIV:
> +	case APIC_ESR:
> +	case APIC_ICR:
> +	case APIC_LVTT:
> +	case APIC_LVTTHMR:
> +	case APIC_LVTPC:
> +	case APIC_LVT0:
> +	case APIC_LVT1:
> +	case APIC_LVTERR:
> +	case APIC_TMICT:
> +	case APIC_TDCR: {

(Try a helper that returns true/false for trap/fault registers, the code
 might look nicer.)

> +		/* Handling Trap */
> +		if (!rw) /* Trap read should never happens */
> +			BUG();
> +		ret = avic_noaccel_trap_write(svm);
> +		break;
> +	}
> +	default: {
> +		/* Handling Fault */
> +		if (rw)
> +			ret = avic_noaccel_fault_write(svm);
> +		else
> +			ret = avic_noaccel_fault_read(svm);
> +		skip_emulated_instruction(&svm->vcpu);

AVIC doesn't tell us what it wanted to write, so KVM has to emulate the
instruction.