From: Paolo Bonzini <pbonzini@redhat.com>
To: linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc: rkrcmar@redhat.com, Vitaly Kuznetsov <vkuznets@redhat.com>,
Junaid Shahid <junaids@google.com>,
Xiao Guangrong <xiaoguangrong@tencent.com>
Subject: [PATCH] KVM: try __get_user_pages_fast even if not in atomic context
Date: Fri, 27 Jul 2018 17:46:47 +0200 [thread overview]
Message-ID: <1532706407-11380-1-git-send-email-pbonzini@redhat.com> (raw)
We are currently cutting hva_to_pfn_fast short if we do not want an
immediate exit, which is represented by !async && !atomic. However,
this is unnecessary, and __get_user_pages_fast is *much* faster
because the regular get_user_pages takes pmd_lock/pte_lock.
In fact, when many CPUs take a nested vmexit at the same time
the contention on those locks is visible, and this patch removes
about 25% (compared to 4.18) from vmexit.flat on a 16 vCPU
nested guest.
Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
virt/kvm/kvm_main.c | 14 ++++++--------
1 file changed, 6 insertions(+), 8 deletions(-)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 861bb20e8451..0f26ff7ddedb 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1343,18 +1343,16 @@ static inline int check_user_page_hwpoison(unsigned long addr)
}
/*
- * The atomic path to get the writable pfn which will be stored in @pfn,
- * true indicates success, otherwise false is returned.
+ * The fast path to get the writable pfn which will be stored in @pfn,
+ * true indicates success, otherwise false is returned. It's also the
+ * only part that runs if we can are in atomic context.
*/
-static bool hva_to_pfn_fast(unsigned long addr, bool atomic, bool *async,
- bool write_fault, bool *writable, kvm_pfn_t *pfn)
+static bool hva_to_pfn_fast(unsigned long addr, bool write_fault,
+ bool *writable, kvm_pfn_t *pfn)
{
struct page *page[1];
int npages;
- if (!(async || atomic))
- return false;
-
/*
* Fast pin a writable pfn only if it is a write fault request
* or the caller allows to map a writable pfn for a read fault
@@ -1498,7 +1496,7 @@ static kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
/* we can do it either atomically or asynchronously, not both */
BUG_ON(atomic && async);
- if (hva_to_pfn_fast(addr, atomic, async, write_fault, writable, &pfn))
+ if (hva_to_pfn_fast(addr, write_fault, writable, &pfn))
return pfn;
if (atomic)
--
1.8.3.1
next reply other threads:[~2018-07-27 15:46 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-27 15:46 Paolo Bonzini [this message]
2018-07-30 8:40 ` [PATCH] KVM: try __get_user_pages_fast even if not in atomic context David Hildenbrand
2018-08-06 7:51 ` Xiao Guangrong
2018-08-06 11:44 ` Paolo Bonzini
2018-08-06 17:39 ` Andrea Arcangeli
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