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Re: [Xen-devel] [PATCH 2/2] x86/tlb: use Xen L0 assisted TLB flush when available
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 27 Dec 2019 14:55:59 +0000
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- Cc: Wei Liu <wl@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>
- Delivery-date: Fri, 27 Dec 2019 14:56:24 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 24/12/2019 13:26, Roger Pau Monne wrote:
> Use Xen's L0 HVMOP_flush_tlbs hypercall when available 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 -j342 xen` in
> shim mode with 32 vCPUs.
>
> Using x2APIC and ALLBUT shorthand:
> real 4m35.973s
> user 4m35.110s
> sys 36m24.117s
>
> Using L0 assisted flush:
> real 1m17.391s
> user 4m42.413s
> sys 6m20.773s
Nice stats.
>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> xen/arch/x86/guest/xen/xen.c | 11 +++++++++++
> xen/arch/x86/smp.c | 6 ++++++
> xen/include/asm-x86/guest/xen.h | 7 +++++++
> 3 files changed, 24 insertions(+)
>
> diff --git a/xen/arch/x86/guest/xen/xen.c b/xen/arch/x86/guest/xen/xen.c
> index 6dbc5f953f..e6493caecf 100644
> --- a/xen/arch/x86/guest/xen/xen.c
> +++ b/xen/arch/x86/guest/xen/xen.c
> @@ -281,6 +281,17 @@ int xg_free_unused_page(mfn_t mfn)
> return rangeset_remove_range(mem, mfn_x(mfn), mfn_x(mfn));
> }
>
> +int xg_flush_tlbs(void)
> +{
> + int rc;
> +
> + do {
> + rc = xen_hypercall_hvm_op(HVMOP_flush_tlbs, NULL);
> + } while ( rc == -ERESTART );
ERESTART should never manifest like this, because it is taken care of
within the hypercall_page[] stub. Anything else is a bug which needs
fixing at L0.
Have you actually seen one appearing?
~Andrew
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