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x86/kvm: Update the comment about asynchronous page fault in exc_page_fault()

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KVM was switched to interrupt-based mechanism for 'page ready' event
delivery in Linux-5.8 (see commit 2635b5c4a0 ("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 b1d405751c ("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: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20201002154313.1505327-1-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Vitaly Kuznetsov 2020-10-02 17:43:13 +02:00 committed by Paolo Bonzini
parent 8f116a6c73
commit 66af4f5cb1
1 changed files with 8 additions and 5 deletions
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13
arch/x86/mm/fault.c
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@ -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