linux-kernel - Re: [PATCH RESEND] KVM: X86: Use IPI shorthands in kvm guest when support

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Message-ID: <20190729175044.GI21120@linux.intel.com>
Date:   Mon, 29 Jul 2019 10:50:45 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nadav Amit <namit@...are.com>
Subject: Re: [PATCH RESEND] KVM: X86: Use IPI shorthands in kvm guest when
 support

On Fri, Jul 26, 2019 at 02:15:27PM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> IPI shorthand is supported now by linux apic/x2apic driver, switch to 
> IPI shorthand for all excluding self and all including self destination 
> shorthand in kvm guest, to avoid splitting the target mask into serveral 
> PV IPI hypercalls.
> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Sean Christopherson <sean.j.christopherson@...el.com>
> Cc: Nadav Amit <namit@...are.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
> Note: rebase against tip tree's x86/apic branch
> 
>  arch/x86/kernel/kvm.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index b7f34fe..87b73b8 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -34,7 +34,9 @@
>  #include <asm/hypervisor.h>
>  #include <asm/tlb.h>
>  
> +static struct apic orig_apic;

Copying the entire struct apic to snapshot two functions is funky,
explicitly capturing the two functions would be more intuitive.

Tangentially related, kvm_setup_pv_ipi() can be annotated __init,
which means the function snapshots can be __ro_after_init.

That being said, can't we just remove kvm_send_ipi_all() and
kvm_send_ipi_allbutself()?  AFAICT, the variations that lead to shorthand
are only used when apic_use_ipi_shorthand is true.  If that isn't the
case, fixing the callers in the APIC code seems like the correct thing to
do.

>  static int kvmapf = 1;
> +DECLARE_STATIC_KEY_FALSE(apic_use_ipi_shorthand);
>  static int __init parse_no_kvmapf(char *arg)
>  {
> @@ -507,12 +509,18 @@ static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int vector)
>  
>  static void kvm_send_ipi_allbutself(int vector)
>  {
> -	kvm_send_ipi_mask_allbutself(cpu_online_mask, vector);
> +	if (static_branch_likely(&apic_use_ipi_shorthand))
> +		orig_apic.send_IPI_allbutself(vector);
> +	else
> +		kvm_send_ipi_mask_allbutself(cpu_online_mask, vector);
>  }
>  
>  static void kvm_send_ipi_all(int vector)
>  {
> -	__send_ipi_mask(cpu_online_mask, vector);
> +	if (static_branch_likely(&apic_use_ipi_shorthand))
> +		orig_apic.send_IPI_all(vector);
> +	else
> +		__send_ipi_mask(cpu_online_mask, vector);
>  }
>  
>  /*
> @@ -520,6 +528,8 @@ static void kvm_send_ipi_all(int vector)
>   */
>  static void kvm_setup_pv_ipi(void)
>  {
> +	orig_apic = *apic;
> +
>  	apic->send_IPI_mask = kvm_send_ipi_mask;
>  	apic->send_IPI_mask_allbutself = kvm_send_ipi_mask_allbutself;
>  	apic->send_IPI_allbutself = kvm_send_ipi_allbutself;
> -- 
> 2.7.4
>