linux-kernel - [tip:x86/urgent] x86/hyperv: Clear vCPU banks between calls to avoid flushing unneeded vCPUs

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Date:   Tue, 10 Oct 2017 07:57:17 -0700
From:   tip-bot for Vitaly Kuznetsov <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, kys@...rosoft.com,
        torvalds@...ux-foundation.org, haiyangz@...rosoft.com,
        tglx@...utronix.de, vkuznets@...hat.com, decui@...rosoft.com,
        peterz@...radead.org, Jork.Loeser@...rosoft.com,
        sthemmin@...rosoft.com, hpa@...or.com
Subject: [tip:x86/urgent] x86/hyperv: Clear vCPU banks between calls to
 avoid flushing unneeded vCPUs

Commit-ID:  a3b7424392924e778b608e30ee321f7b10cc94b8
Gitweb:     https://git.kernel.org/tip/a3b7424392924e778b608e30ee321f7b10cc94b8
Author:     Vitaly Kuznetsov <vkuznets@...hat.com>
AuthorDate: Fri, 6 Oct 2017 17:48:54 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 10 Oct 2017 12:53:00 +0200

x86/hyperv: Clear vCPU banks between calls to avoid flushing unneeded vCPUs

hv_flush_pcpu_ex structures are not cleared between calls for performance
reasons (they're variable size up to PAGE_SIZE each) but we must clear
hv_vp_set.bank_contents part of it to avoid flushing unneeded vCPUs. The
rest of the structure is formed correctly.

To do the clearing in an efficient way stash the maximum possible vCPU
number (this may differ from Linux CPU id).

Reported-by: Jork Loeser <Jork.Loeser@...rosoft.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: Dexuan Cui <decui@...rosoft.com>
Cc: Haiyang Zhang <haiyangz@...rosoft.com>
Cc: K. Y. Srinivasan <kys@...rosoft.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Stephen Hemminger <sthemmin@...rosoft.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: devel@...uxdriverproject.org
Link: http://lkml.kernel.org/r/20171006154854.18092-1-vkuznets@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/hyperv/hv_init.c       |  5 +++++
 arch/x86/hyperv/mmu.c           | 17 ++++++++++++-----
 arch/x86/include/asm/mshyperv.h |  1 +
 3 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 1a8eb55..a5db63f 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -85,6 +85,8 @@ EXPORT_SYMBOL_GPL(hyperv_cs);
 u32 *hv_vp_index;
 EXPORT_SYMBOL_GPL(hv_vp_index);
 
+u32 hv_max_vp_index;
+
 static int hv_cpu_init(unsigned int cpu)
 {
 	u64 msr_vp_index;
@@ -93,6 +95,9 @@ static int hv_cpu_init(unsigned int cpu)
 
 	hv_vp_index[smp_processor_id()] = msr_vp_index;
 
+	if (msr_vp_index > hv_max_vp_index)
+		hv_max_vp_index = msr_vp_index;
+
 	return 0;
 }
 
diff --git a/arch/x86/hyperv/mmu.c b/arch/x86/hyperv/mmu.c
index 39e7f6e..9502d04 100644
--- a/arch/x86/hyperv/mmu.c
+++ b/arch/x86/hyperv/mmu.c
@@ -76,6 +76,18 @@ static inline int cpumask_to_vp_set(struct hv_flush_pcpu_ex *flush,
 {
 	int cpu, vcpu, vcpu_bank, vcpu_offset, nr_bank = 1;
 
+	/* valid_bank_mask can represent up to 64 banks */
+	if (hv_max_vp_index / 64 >= 64)
+		return 0;
+
+	/*
+	 * Clear all banks up to the maximum possible bank as hv_flush_pcpu_ex
+	 * structs are not cleared between calls, we risk flushing unneeded
+	 * vCPUs otherwise.
+	 */
+	for (vcpu_bank = 0; vcpu_bank <= hv_max_vp_index / 64; vcpu_bank++)
+		flush->hv_vp_set.bank_contents[vcpu_bank] = 0;
+
 	/*
 	 * Some banks may end up being empty but this is acceptable.
 	 */
@@ -83,11 +95,6 @@ static inline int cpumask_to_vp_set(struct hv_flush_pcpu_ex *flush,
 		vcpu = hv_cpu_number_to_vp_number(cpu);
 		vcpu_bank = vcpu / 64;
 		vcpu_offset = vcpu % 64;
-
-		/* valid_bank_mask can represent up to 64 banks */
-		if (vcpu_bank >= 64)
-			return 0;
-
 		__set_bit(vcpu_offset, (unsigned long *)
 			  &flush->hv_vp_set.bank_contents[vcpu_bank]);
 		if (vcpu_bank >= nr_bank)
diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
index 738503e..530f448 100644
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -289,6 +289,7 @@ static inline u64 hv_do_rep_hypercall(u16 code, u16 rep_count, u16 varhead_size,
  * to this information.
  */
 extern u32 *hv_vp_index;
+extern u32 hv_max_vp_index;
 
 /**
  * hv_cpu_number_to_vp_number() - Map CPU to VP.