linux-kernel - Re: [PART1 RFC v3 12/12] svm: Manage vcpu load/unload when enable AVIC

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Message-ID: <20160318214450.GB2332@potion.brq.redhat.com>
Date:	Fri, 18 Mar 2016 22:44:51 +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 v3 12/12] svm: Manage vcpu load/unload when enable
 AVIC

2016-03-18 01:09-0500, Suravee Suthikulpanit:
> From: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
> 
> When a vcpu is loaded/unloaded to a physical core, we need to update
> host physical APIC ID information in the Physical APIC-ID table
> accordingly.
> 
> Also, when vCPU is blocking/un-blocking (due to halt instruction),
> we need to make sure that the is-running bit in set accordingly in the
> physical APIC-ID table.
> 
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
> ---
>  arch/x86/kvm/svm.c | 121 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 121 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> +static int avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu, bool is_load)
> +{
> +	int h_phy_apic_id;

(Paolo said a lot about those names.)

> +	u64 *entry, new_entry;
> +	struct vcpu_svm *svm = to_svm(vcpu);
> +	int ret = 0;
> +	if (!svm_vcpu_avic_enabled(svm))
> +		return 0;

(The check for NULL below feels weird when it has already been used.)

> +
> +	if (!svm)
> +		return -EINVAL;

!svm means that KVM completely blew up ... don't check for it.
(See implementation of to_svm.)

> +
> +	/* Note: APIC ID = 0xff is used for broadcast.
> +	 *       APIC ID > 0xff is reserved.
> +	 */
> +	h_phy_apic_id = __default_cpu_present_to_apicid(cpu);
> +
> +	if (h_phy_apic_id >= AVIC_PHY_APIC_ID_MAX)
> +		return -EINVAL;
> +
> +	entry = svm->avic_phy_apic_id_cache;

The naming is confusing ... can avic_phy_apic_id_cache change during
execution of this function?
If yes, then add READ_ONCE and distinguish the pointer name.
If not, then use svm->avic_phy_apic_id_cache directly.

entry would be ok name for current new_entry.

> +	if (!entry)
> +		return -EINVAL;
> +
> +	if (is_load) {
> +		new_entry = READ_ONCE(*entry);

Please move this before the if.

> +		BUG_ON(new_entry & AVIC_PHY_APIC_ID__IS_RUN_MSK);
> +
> +		new_entry &= ~AVIC_PHY_APIC_ID__HOST_PHY_APIC_ID_MSK;
> +		new_entry |= (h_phy_apic_id & AVIC_PHY_APIC_ID__HOST_PHY_APIC_ID_MSK);
> +
> +		/**
> +		 * Restore AVIC running flag if it was set during
> +		 * vcpu unload.
> +		 */
> +		if (svm->avic_was_running)
> +			new_entry |= AVIC_PHY_APIC_ID__IS_RUN_MSK;
> +		else
> +			new_entry &= ~AVIC_PHY_APIC_ID__IS_RUN_MSK;

You even BUG_ON when AVIC_PHY_APIC_ID__IS_RUN_MSK is set, so there is no
reason to clear it.

(Also, don't BUG.)

> +
> +		WRITE_ONCE(*entry, new_entry);

This will translate to two writes in 32 bit mode and we need to write
physical ID first to avoid spurious doorbells ...
is the order guaranteed?

> +	} else {
> +		new_entry = READ_ONCE(*entry);
> +		/**
> +		 * This handles the case when vcpu is scheduled out
> +		 * and has not yet not called blocking. We save the
> +		 * AVIC running flag so that we can restore later.
> +		 */

is_running must be disabled in between ...blocking and ...unblocking,
because we don't want to miss interrupts and block forever.
I somehow don't get it from the comment. :)

> +		if (new_entry & AVIC_PHY_APIC_ID__IS_RUN_MSK) {
> +			svm->avic_was_running = true;
> +			new_entry &= ~AVIC_PHY_APIC_ID__IS_RUN_MSK;
> +			WRITE_ONCE(*entry, new_entry);
> +		} else {
> +			svm->avic_was_running = false;
> +		}

(This can be shorter by setting avic_was_running first.)

> +	}
> +
> +	return ret;

(return 0;)

> +}
> +
> +/**
> + * This function is called during VCPU halt/unhalt.
> + */
> +static int avic_set_running(struct kvm_vcpu *vcpu, bool is_run)
> +{
> +	int ret = 0;
> +	int h_phy_apic_id;
> +	u64 *entry, new_entry;
> +	struct vcpu_svm *svm = to_svm(vcpu);
> +
> +	if (!svm_vcpu_avic_enabled(svm))
> +		return 0;
> +
> +	/* Note: APIC ID = 0xff is used for broadcast.
> +	 *       APIC ID > 0xff is reserved.
> +	 */
> +	h_phy_apic_id = __default_cpu_present_to_apicid(vcpu->cpu);
> +
> +	if (h_phy_apic_id >= AVIC_PHY_APIC_ID_MAX)
> +		return -EINVAL;

The cache should be valid only if this condition is true.
We can get rid of it in both function.

> +
> +	entry = svm->avic_phy_apic_id_cache;
> +	if (!entry)
> +		return -EINVAL;
> +
> +	if (is_run) {

Both READ_ONCE and WRITE_ONCE belong outside of the if.

> +		/* Handle vcpu unblocking after HLT */
> +		new_entry = READ_ONCE(*entry);
> +		new_entry |= AVIC_PHY_APIC_ID__IS_RUN_MSK;
> +		WRITE_ONCE(*entry, new_entry);
> +	} else {
> +		/* Handle vcpu blocking due to HLT */
> +		new_entry = READ_ONCE(*entry);
> +		new_entry &= ~AVIC_PHY_APIC_ID__IS_RUN_MSK;
> +		WRITE_ONCE(*entry, new_entry);
> +	}
> +
> +	return ret;
> +}
> +
>  static void svm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);