linux-kernel - Re: [PATCH v2 14/15] kvm: ioapic: Delay update IOAPIC EOI for RTC

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Date:   Mon, 19 Aug 2019 13:00:07 +0200
From:   Alexander Graf <graf@...zon.com>
To:     "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "jschoenh@...zon.de" <jschoenh@...zon.de>,
        "karahmed@...zon.de" <karahmed@...zon.de>,
        "rimasluk@...zon.com" <rimasluk@...zon.com>,
        "Grimm, Jon" <Jon.Grimm@....com>
Subject: Re: [PATCH v2 14/15] kvm: ioapic: Delay update IOAPIC EOI for RTC



On 15.08.19 18:25, Suthikulpanit, Suravee wrote:
> In-kernel IOAPIC does not update RTC pending EOI info with AMD SVM /w AVIC
> when interrupt is delivered as edge-triggered since AMD processors
> cannot exit on EOI for these interrupts.
> 
> Add code to also check LAPIC pending EOI before injecting any new RTC
> interrupts on AMD SVM when AVIC is activated.
> 
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
> ---
>   arch/x86/kvm/ioapic.c | 36 ++++++++++++++++++++++++++++++++----
>   1 file changed, 32 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c
> index 1add1bc..45e7bb0 100644
> --- a/arch/x86/kvm/ioapic.c
> +++ b/arch/x86/kvm/ioapic.c
> @@ -39,6 +39,7 @@
>   #include <asm/processor.h>
>   #include <asm/page.h>
>   #include <asm/current.h>
> +#include <asm/virtext.h>
>   #include <trace/events/kvm.h>
>   
>   #include "ioapic.h"
> @@ -173,6 +174,7 @@ static bool rtc_irq_check_coalesced(struct kvm_ioapic *ioapic)
>   	return false;
>   }
>   
> +#define APIC_DEST_NOSHORT		0x0
>   static int ioapic_set_irq(struct kvm_ioapic *ioapic, unsigned int irq,
>   		int irq_level, bool line_status)
>   {
> @@ -201,10 +203,36 @@ static int ioapic_set_irq(struct kvm_ioapic *ioapic, unsigned int irq,
>   	 * interrupts lead to time drift in Windows guests.  So we track
>   	 * EOI manually for the RTC interrupt.
>   	 */
> -	if (irq == RTC_GSI && line_status &&
> -		rtc_irq_check_coalesced(ioapic)) {
> -		ret = 0;
> -		goto out;
> +	if (irq == RTC_GSI && line_status) {
> +		struct kvm *kvm = ioapic->kvm;
> +		union kvm_ioapic_redirect_entry *entry = &ioapic->redirtbl[irq];
> +
> +		/*
> +		 * Since, AMD SVM AVIC accelerates write access to APIC EOI
> +		 * register for edge-trigger interrupts, IOAPIC will not be
> +		 * able to receive the EOI. In this case, we do lazy update
> +		 * of the pending EOI when trying to set IOAPIC irq for RTC.
> +		 */
> +		if (cpu_has_svm(NULL) &&
> +		    (kvm->arch.apicv_state == APICV_ACTIVATED) &&
> +		    (entry->fields.trig_mode == IOAPIC_EDGE_TRIG)) {
> +			int i;
> +			struct kvm_vcpu *vcpu;
> +
> +			kvm_for_each_vcpu(i, vcpu, kvm)
> +				if (kvm_apic_match_dest(vcpu, NULL,
> +							KVM_APIC_DEST_NOSHORT,
> +							entry->fields.dest_id,
> +							entry->fields.dest_mode)) {
> +					__rtc_irq_eoi_tracking_restore_one(vcpu);

I don't understand why this works. This code just means we're injecting 
an EOI on the first CPU that has the vector mapped, right when we're 
setting it to trigger, no?


Alex


> +					break;
> +				}
> +		}
> +
> +		if (rtc_irq_check_coalesced(ioapic)) {
> +			ret = 0;
> +			goto out;
> +		}
>   	}
>   
>   	old_irr = ioapic->irr;
>