Re: [Xen-devel] [PATCH v4 7/7] x86/tlb: use Xen L0 assisted TLB flush when available

[Wei Liu <wl@xxxxxxx>]

On Mon, Feb 10, 2020 at 06:28:29PM +0100, Roger Pau Monne wrote:
> Use Xen's L0 HVMOP_flush_tlbs hypercall in order to perform flushes.
> This greatly increases the performance of TLB flushes when running
> with a high amount of vCPUs as a Xen guest, and is specially important
> when running in shim mode.
> 
> The following figures are from a PV guest running `make -j32 xen` in
> shim mode with 32 vCPUs and HAP.
> 
> Using x2APIC and ALLBUT shorthand:
> real  4m35.973s
> user  4m35.110s
> sys   36m24.117s
> 
> Using L0 assisted flush:
> real    1m2.596s
> user    4m34.818s
> sys     5m16.374s
> 
> The implementation adds a new hook to hypervisor_ops so other
> enlightenments can also implement such assisted flush just by filling
> the hook. Note that the Xen implementation completely ignores the
> dirty CPU mask and the linear address passed in, and always performs a
> global TLB flush on all vCPUs.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Wei Liu <wl@xxxxxxx>

One remark below.

[...]
>  const struct hypervisor_ops *__init xg_probe(void)
> diff --git a/xen/arch/x86/smp.c b/xen/arch/x86/smp.c
> index 65eb7cbda8..9bc925616a 100644
> --- a/xen/arch/x86/smp.c
> +++ b/xen/arch/x86/smp.c
> @@ -15,6 +15,7 @@
>  #include <xen/perfc.h>
>  #include <xen/spinlock.h>
>  #include <asm/current.h>
> +#include <asm/guest.h>
>  #include <asm/smp.h>
>  #include <asm/mc146818rtc.h>
>  #include <asm/flushtlb.h>
> @@ -256,6 +257,16 @@ void flush_area_mask(const cpumask_t *mask, const void 
> *va, unsigned int flags)
>      if ( (flags & ~FLUSH_ORDER_MASK) &&
>           !cpumask_subset(mask, cpumask_of(cpu)) )
>      {
> +        if ( cpu_has_hypervisor &&
> +             !(flags & ~(FLUSH_TLB | FLUSH_TLB_GLOBAL | FLUSH_VA_VALID |
> +                         FLUSH_ORDER_MASK)) &&
> +             !hypervisor_flush_tlb(mask, va, flags & FLUSH_ORDER_MASK) )

I would like pass in the flag as a whole because Hyper-V has the
capability to fine tune what gets flushed. I can see FLUSH_TLB_GLOBAL
being used in that situation.

There is no need to change your patch though. I can submit a patch
myself to change this interface once your series is accepted.

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