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

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

[Vitaly Kuznetsov]

KVM was switched to interrupt-based mechanism for 'page ready' event
delivery in Linux-5.8 (see commit 2635b5c4a0e4 ("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 b1d405751cd5 ("KVM: x86: Switch KVM guest to
using interrupts for page ready APF delivery")) so it is not possible to
get 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>
---
 arch/x86/mm/fault.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 6e3e8a124903..3cf77592ac54 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -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
-- 
2.25.4

Re: [PATCH] x86/kvm: Update the comment about asynchronous page fault in exc_page_fault()

[Vitaly Kuznetsov]

Vitaly Kuznetsov <vkuznets@redhat.com> writes:

> KVM was switched to interrupt-based mechanism for 'page ready' event
> delivery in Linux-5.8 (see commit 2635b5c4a0e4 ("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 b1d405751cd5 ("KVM: x86: Switch KVM guest to
> using interrupts for page ready APF delivery")) so it is not possible to
> get older KVM (APF mechanism won't be enabled). Update the comment in
   
Sigh, this should have been

"to get #PF for a 'page ready' event even when the guest is running on
top of an older KVM"

or something like that but the part after "#" was eaten by git :-(

> exc_page_fault() to reflect the new reality.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  arch/x86/mm/fault.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 6e3e8a124903..3cf77592ac54 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -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

-- 
Vitaly

Re: [PATCH] x86/kvm: Update the comment about asynchronous page fault in exc_page_fault()

[Paolo Bonzini]

On 02/10/20 17:43, Vitaly Kuznetsov wrote:
> KVM was switched to interrupt-based mechanism for 'page ready' event
> delivery in Linux-5.8 (see commit 2635b5c4a0e4 ("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 b1d405751cd5 ("KVM: x86: Switch KVM guest to
> using interrupts for page ready APF delivery")) so it is not possible to
> get 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>
> ---
>  arch/x86/mm/fault.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 6e3e8a124903..3cf77592ac54 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -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
> 

Queued, thanks.

Paolo