Re: [PATCH] enable x2APIC without interrupt remapping under KVM

Sheng Yang
2009-06-29T07:54:10Z

On Sunday 28 June 2009 20:51:14 Gleb Natapov 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).

[Resend, sorry for html noise...]

Add Suresh here. And some comments and unclear points below.

> Signed-off-by: Gleb Natapov <gleb@redhat.com>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index d1430ef..7438c5c 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -260,7 +260,7 @@ config SMP
>
> config X86_X2APIC
> bool "Support x2apic"
> - depends on X86_LOCAL_APIC && X86_64 && INTR_REMAP
> + depends on X86_LOCAL_APIC && X86_64
> ---help---
> This enables x2apic support on CPUs that have this feature.

We may need to note x2apic won't work without interrupt remapping or as a
guest in KVM here?
>
> diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
> index 8c7c042..48a63b6 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;
>
> @@ -1363,11 +1364,10 @@ 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();
> @@ -1381,11 +1381,10 @@ void __init enable_IR_x2apic(void)
> goto ir_failed;
> }
>
> -
> if (!x2apic_preenabled && skip_ioapic_setup) {
> pr_info("Skipped enabling intr-remap because of skipping "
> "io-apic setup\n");
> - return;
> + goto ir_failed;
> }
>
> ioapic_entries = alloc_ioapic_entries();
> @@ -1400,22 +1399,14 @@ void __init enable_IR_x2apic(void)
> goto end;
> }
>
> - local_irq_save(flags);
> mask_IO_APIC_setup(ioapic_entries);
> - mask_8259A();
>
> ret = enable_intr_remapping(x2apic_supported());
> if (ret)
> goto end_restore;
> -

Unrelated...

> pr_info("Enabled Interrupt-remapping\n");
>
> - if (x2apic_supported() && !x2apic_mode) {
> - x2apic_mode = 1;
> - enable_x2apic();
> - pr_info("Enabled x2apic\n");
> - }
> -
> + return 1;
> end_restore:
> if (ret)
> /*
> @@ -1423,30 +1414,50 @@ end_restore:
> */
> restore_IO_APIC_setup(ioapic_entries);
>
> - unmask_8259A();
> - local_irq_restore(flags);
> -
> end:
> if (ioapic_entries)
> free_ioapic_entries(ioapic_entries);
>
> - if (!ret)
> - return;
> -
> ir_failed:
> - if (x2apic_preenabled)
> +#endif
> + return 0;
> +}
> +
> +void __init enable_IR_x2apic(void)
> +{
> + unsigned long flags;
> +
> + local_irq_save(flags);
> + mask_8259A();
> +
> + /* IR is required if x2apic is enabled by BIOS even when running in kvm
> + * since this indicates present of APIC ID > 255 */
> + if (!enable_IR() && (x2apic_preenabled || !kvm_para_available()))
> + goto nox2apic;
> +

Not quite understand the comment here. Could you explain why "since this
indicates present of APIC ID > 255?" In another word, why enable_IR() needed
for KVM even it would be fail at dmar_table_init()?

Another question is, we supposed KVM would use physical mode of x2apic, for
IOAPIC and MSI won't support APIC ID > 255. But seems not, or I miss
something?

--
regards
Yang, Sheng

> + if (x2apic_supported() && !x2apic_mode) {
> + x2apic_mode = 1;
> + enable_x2apic();
> + pr_info("Enabled x2apic\n");
> + }
> +
> + unmask_8259A();
> + local_irq_restore(flags);
> + return;
> +
> +nox2apic:
> + unmask_8259A();
> + local_irq_restore(flags);
> +
> + if (x2apic_preenabled) {
> +#ifdef CONFIG_INTR_REMAP
> panic("x2apic enabled by bios. But IR enabling failed");
> - else if (cpu_has_x2apic)
> - pr_info("Not enabling x2apic,Intr-remapping\n");
> #else
> - if (!cpu_has_x2apic)
> - return;
> -
> - if (x2apic_preenabled)
> panic("x2apic enabled prior OS handover,"
> " enable CONFIG_X86_X2APIC, CONFIG_INTR_REMAP");
> #endif
> -
> + } else if (cpu_has_x2apic)
> + pr_info("Not enabling x2apic,Intr-remapping\n");
> return;
> }
>
> --
> Gleb.
> --
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