Re: [Xen-devel] [PATCH v2 4/9] x86/mm/tlb: Flush remote and local TLBs concurrently

On Jul 3, 2019, at 10:43 AM, Andrew Cooper [off-list ref] wrote:

On 03/07/2019 18:02, Nadav Amit wrote:

quoted

quoted

On Jul 3, 2019, at 7:04 AM, Juergen Gross [off-list ref] wrote:

On 03.07.19 01:51, Nadav Amit wrote:

quoted

To improve TLB shootdown performance, flush the remote and local TLBs
concurrently. Introduce flush_tlb_multi() that does so. Introduce
paravirtual versions of flush_tlb_multi() for KVM, Xen and hyper-v (Xen
and hyper-v are only compile-tested).
While the updated smp infrastructure is capable of running a function on
a single local core, it is not optimized for this case. The multiple
function calls and the indirect branch introduce some overhead, and
might make local TLB flushes slower than they were before the recent
changes.
Before calling the SMP infrastructure, check if only a local TLB flush
is needed to restore the lost performance in this common case. This
requires to check mm_cpumask() one more time, but unless this mask is
updated very frequently, this should impact performance negatively.
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Stephen Hemminger <redacted>
Cc: Sasha Levin <sashal@kernel.org>
Cc: Thomas Gleixner <redacted>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: x86@kernel.org
Cc: Juergen Gross <jgross@suse.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: linux-hyperv@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: virtualization@lists.linux-foundation.org
Cc: kvm@vger.kernel.org
Cc: xen-devel@lists.xenproject.org
Signed-off-by: Nadav Amit <redacted>
---
arch/x86/hyperv/mmu.c                 | 13 +++---
arch/x86/include/asm/paravirt.h       |  6 +--
arch/x86/include/asm/paravirt_types.h |  4 +-
arch/x86/include/asm/tlbflush.h       |  9 ++--
arch/x86/include/asm/trace/hyperv.h   |  2 +-
arch/x86/kernel/kvm.c                 | 11 +++--
arch/x86/kernel/paravirt.c            |  2 +-
arch/x86/mm/tlb.c                     | 65 ++++++++++++++++++++-------
arch/x86/xen/mmu_pv.c                 | 20 ++++++---
include/trace/events/xen.h            |  2 +-
10 files changed, 91 insertions(+), 43 deletions(-)

...

quoted

diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
index beb44e22afdf..19e481e6e904 100644
--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c

@@ -1355,8 +1355,8 @@ static void xen_flush_tlb_one_user(unsigned long addr)

	preempt_enable();
}
-static void xen_flush_tlb_others(const struct cpumask *cpus,
-				 const struct flush_tlb_info *info)
+static void xen_flush_tlb_multi(const struct cpumask *cpus,
+				const struct flush_tlb_info *info)
{
	struct {
		struct mmuext_op op;

@@ -1366,7 +1366,7 @@ static void xen_flush_tlb_others(const struct cpumask *cpus,

	const size_t mc_entry_size = sizeof(args->op) +
		sizeof(args->mask[0]) * BITS_TO_LONGS(num_possible_cpus());
-	trace_xen_mmu_flush_tlb_others(cpus, info->mm, info->start, info->end);
+	trace_xen_mmu_flush_tlb_multi(cpus, info->mm, info->start, info->end);
  	if (cpumask_empty(cpus))
		return;		/* nothing to do */

@@ -1375,9 +1375,17 @@ static void xen_flush_tlb_others(const struct cpumask *cpus,

	args = mcs.args;
	args->op.arg2.vcpumask = to_cpumask(args->mask);
-	/* Remove us, and any offline CPUS. */
+	/* Flush locally if needed and remove us */
+	if (cpumask_test_cpu(smp_processor_id(), to_cpumask(args->mask))) {
+		local_irq_disable();
+		flush_tlb_func_local(info);

I think this isn't the correct function for PV guests.

In fact it should be much easier: just don't clear the own cpu from the
mask, that's all what's needed. The hypervisor is just fine having the
current cpu in the mask and it will do the right thing.

Thanks. I will do so in v3. I don’t think Hyper-V people would want to do
the same, unfortunately, since it would induce VM-exit on TLB flushes.

Why do you believe the vmexit matters?  You're talking one anyway for
the IPI.

Intel only have virtualised self-IPI, and while AMD do have working
non-self IPIs, you still take a vmexit anyway if any destination vcpu
isn't currently running in non-root mode (IIRC).

At that point, you might as well have the hypervisor do all the hard
work via a multi-cpu shootdown/flush hypercall, rather than trying to
arrange it locally.

I forgot that xen_flush_tlb_multi() should actually only be called when
there are some remote CPUs (as I optimized the case in which there is only a
single local CPU that needs to be flushed), so you are right.

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