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Commit b9b33da2 authored by Paolo Bonzini's avatar Paolo Bonzini
Browse files

KVM: try __get_user_pages_fast even if not in atomic context 



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: default avatarAndrea Arcangeli <aarcange@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 877ad952

virt/kvm/kvm_main.c

+6 −8
Original line number Diff line number Diff line
@@ -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)