linux-kernel - Re: [PATCH v4] enable x2APIC without interrupt remapping under KVM

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Message-ID: <86802c440906300018p8c5156dy3e8d84b8c263797e@mail.gmail.com>
Date:	Tue, 30 Jun 2009 00:18:19 -0700
From:	Yinghai Lu <yhlu.kernel@...il.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Sheng Yang <sheng@...ux.intel.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"avi@...hat.com" <avi@...hat.com>
Subject: Re: [PATCH v4] enable x2APIC without interrupt remapping under KVM

On Mon, Jun 29, 2009 at 11:45 PM, Gleb Natapov<gleb@...hat.com> wrote:
> KVM would like to provide x2APIC interface to a guest without emulating
> interrupt remapping device. The reason KVM prefers guest to use x2APIC
> is that x2APIC interface is better virtualizable and provides better
> performance than mmio xAPIC interface:
>
> - msr exits are faster than mmio (no page table walk, emulation)
> - no need to read back ICR to look at the busy bit
> - one 64 bit ICR write instead of two 32 bit writes
> - shared code with the Hyper-V paravirt interface
>
> Included patch changes x2APIC enabling logic to enable it even if IR
> initialization failed, but kernel runs under KVM and no apic id is
> greater than 255 (if there is one spec requires BIOS to move to x2apic
> mode before starting an OS).
>
> Signed-off-by: Gleb Natapov <gleb@...hat.com>
> diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
> index 8c7c042..351d55a 100644
> --- a/arch/x86/kernel/apic/apic.c
> +++ b/arch/x86/kernel/apic/apic.c
> @@ -49,6 +49,7 @@
>  #include <asm/mtrr.h>
>  #include <asm/smp.h>
>  #include <asm/mce.h>
> +#include <asm/kvm_para.h>
>
>  unsigned int num_processors;
>
> @@ -1363,52 +1364,75 @@ void enable_x2apic(void)
>  }
>  #endif /* CONFIG_X86_X2APIC */
>
> -void __init enable_IR_x2apic(void)
> +int __init enable_IR(void)
>  {
>  #ifdef CONFIG_INTR_REMAP
>        int ret;
> -       unsigned long flags;
> -       struct IO_APIC_route_entry **ioapic_entries = NULL;
>
>        ret = dmar_table_init();
>        if (ret) {
>                pr_debug("dmar_table_init() failed with %d:\n", ret);
> -               goto ir_failed;
> +               return 0;
>        }
>
>        if (!intr_remapping_supported()) {
>                pr_debug("intr-remapping not supported\n");
> -               goto ir_failed;
> +               return 0;
>        }
>
> -
>        if (!x2apic_preenabled && skip_ioapic_setup) {
>                pr_info("Skipped enabling intr-remap because of skipping "
>                        "io-apic setup\n");
> -               return;
> +               return 0;
>        }
>
> +       if (enable_intr_remapping(x2apic_supported()))
> +               return 0;
> +
> +       pr_info("Enabled Interrupt-remapping\n");
> +
> +       return 1;
> +
> +#endif
> +       return 0;
> +}
> +
> +void __init enable_IR_x2apic(void)
> +{
> +       unsigned long flags;
> +       struct IO_APIC_route_entry **ioapic_entries = NULL;
> +       int ret, x2apic_enabled = 0;
> +
>        ioapic_entries = alloc_ioapic_entries();
>        if (!ioapic_entries) {
> -               pr_info("Allocate ioapic_entries failed: %d\n", ret);
> -               goto end;
> +                pr_info("Allocate ioapic_entries failed\n");
> +                goto out;
>        }
>
>        ret = save_IO_APIC_setup(ioapic_entries);
>        if (ret) {
>                pr_info("Saving IO-APIC state failed: %d\n", ret);
> -               goto end;
> +               goto out;
>        }
>
>        local_irq_save(flags);
> -       mask_IO_APIC_setup(ioapic_entries);
>        mask_8259A();
> +       mask_IO_APIC_setup(ioapic_entries);
>
> -       ret = enable_intr_remapping(x2apic_supported());
> -       if (ret)
> -               goto end_restore;
> +       ret = enable_IR();
> +       if (!ret) {
> +               /* IR is required if x2apic is enabled by BIOS even
> +                * when running in kvm since this indicates present
> +                * of APIC ID > 255 */
> +               if (x2apic_preenabled || !kvm_para_available())
> +                       goto nox2apic;
> +               else
> +                       /* without IR all CPUs can be addressed by IOAPIC/MSI
> +                        * only in physical mode */
> +                       x2apic_phys = 1;
> +       }

how about kexec second kernel in KVM ?

x2apic_preenabled will be set in second kernel.

YH
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