Commit 66af4f5c authored by Vitaly Kuznetsov's avatar Vitaly Kuznetsov Committed by Paolo Bonzini
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x86/kvm: Update the comment about asynchronous page fault in exc_page_fault() 



KVM was switched to interrupt-based mechanism for 'page ready' event
delivery in Linux-5.8 (see commit 2635b5c4 ("KVM: x86: interrupt based
APF 'page ready' event delivery")) and #PF (ab)use for 'page ready' event
delivery was removed. Linux guest switched to this new mechanism
exclusively in 5.9 (see commit b1d40575 ("KVM: x86: Switch KVM guest to
using interrupts for page ready APF delivery")) so it is not possible to
get #PF for a 'page ready' event even when the guest is running on top
of an older KVM (APF mechanism won't be enabled). Update the comment in
exc_page_fault() to reflect the new reality.

Signed-off-by: default avatarVitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20201002154313.1505327-1-vkuznets@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 8f116a6c

arch/x86/mm/fault.c

+8 −5
Original line number Diff line number Diff line
@@ -1446,11 +1446,14 @@ DEFINE_IDTENTRY_RAW_ERRORCODE(exc_page_fault) 
	prefetchw(&current->mm->mmap_lock);



	/*

	 * KVM has two types of events that are, logically, interrupts, but

	 * are unfortunately delivered using the #PF vector.  These events are

	 * "you just accessed valid memory, but the host doesn't have it right

	 * now, so I'll put you to sleep if you continue" and "that memory

	 * you tried to access earlier is available now."

	 * KVM uses #PF vector to deliver 'page not present' events to guests

	 * (asynchronous page fault mechanism). The event happens when a

	 * userspace task is trying to access some valid (from guest's point of

	 * view) memory which is not currently mapped by the host (e.g. the

	 * memory is swapped out). Note, the corresponding "page ready" event

	 * which is injected when the memory becomes available, is delived via

	 * an interrupt mechanism and not a #PF exception

	 * (see arch/x86/kernel/kvm.c: sysvec_kvm_asyncpf_interrupt()).

	 *

	 * We are relying on the interrupted context being sane (valid RSP,

	 * relevant locks not held, etc.), which is fine as long as the