From: Avi Kivity <avi@redhat.com>
To: Gleb Natapov <gleb@redhat.com>
Cc: kvm@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 01/11] Add shared memory hypercall to PV Linux guest.
Date: Tue, 03 Nov 2009 09:40:40 +0200 [thread overview]
Message-ID: <4AEFDE78.5030507@redhat.com> (raw)
In-Reply-To: <20091103071638.GK27911@redhat.com>
On 11/03/2009 09:16 AM, Gleb Natapov wrote:
>>>
>>> I have both! Do you want me to drop version?
>>>
>> Yes. Once a kernel is released you can't realistically change the version.
>>
>>
> Why not? If version doesn't match apf will not be used.
>
Then you cause a large performance regression (assuming apf is any
good). So there will be a lot of pressure to modify things
incrementally via feature bits.
>
>>>> Some documentation for this?
>>>>
>>>> Also, the name should reflect the pv pagefault use. For other uses
>>>> we can register other areas.
>>>>
>>>>
>>> I wanted it to be generic, but I am fine with making it apf specific.
>>> It will allow to make it smaller too.
>>>
>> Maybe we can squeeze it into the page-fault error code?
>>
>>
> apf has to pass two things into a guest kernel:
> - event type (page not present/wake up)
> - unique token
> Error code has 32 bits and at least 1 of them should indicate that this
> is apf another one should indicate event type so this leaves us 30 bits
> for a token. 12 bits of a token is used to store vcpu id this leaves 18
> bits for unique per vcpu id. Yes this may be enough. I don't think it is
> realistic to have more then 200000 outstanding apfs per vcpu. Alternately
> we can use CR2 to pass a token.
>
Or a combination of pfec and cr2, yes.
>>>> would solve this. I prefer using put_user() though than a permanent
>>>> get_user_pages().
>>>>
>>>>
>>> I want to prevent it from been swapped out.
>>>
>> Since you don't prevent the page fault handler or code from being
>> swapped out, you don't get anything out of it.
>>
>>
> Performance. Currently it is accessed on each page fault and to access
> it gup+kmap should be done each and every time.
>
put_user() is just as fast as a kmap, and don't prevent page migration
or defragmentation.
Note we still have to mark_page_dirty() unless we want to chase live
migration bugs.
--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.
next prev parent reply other threads:[~2009-11-03 7:40 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-11-01 11:56 [PATCH 00/11] KVM: Add asynchronous page fault for PV guest Gleb Natapov
2009-11-01 11:56 ` [PATCH 01/11] Add shared memory hypercall to PV Linux guest Gleb Natapov
2009-11-02 4:27 ` Rik van Riel
2009-11-02 7:07 ` Gleb Natapov
2009-11-02 12:18 ` Avi Kivity
2009-11-02 16:18 ` Gleb Natapov
2009-11-03 5:15 ` Avi Kivity
2009-11-03 7:16 ` Gleb Natapov
2009-11-03 7:40 ` Avi Kivity [this message]
2009-11-01 11:56 ` [PATCH 02/11] Add "handle page fault" PV helper Gleb Natapov
2009-11-02 9:22 ` Ingo Molnar
2009-11-02 16:04 ` Gleb Natapov
2009-11-02 16:12 ` Ingo Molnar
2009-11-02 16:22 ` Gleb Natapov
2009-11-02 16:29 ` Ingo Molnar
2009-11-02 16:31 ` Gleb Natapov
2009-11-02 17:42 ` Gleb Natapov
2009-11-08 11:36 ` Ingo Molnar
2009-11-08 12:43 ` Avi Kivity
2009-11-08 12:51 ` Ingo Molnar
2009-11-08 13:01 ` Avi Kivity
2009-11-08 13:05 ` Ingo Molnar
2009-11-08 13:08 ` Avi Kivity
2009-11-08 16:44 ` H. Peter Anvin
2009-11-08 16:47 ` Ingo Molnar
2009-11-02 19:03 ` Rik van Riel
2009-11-02 19:33 ` Avi Kivity
2009-11-02 23:35 ` Rik van Riel
2009-11-03 4:57 ` Avi Kivity
2009-11-05 6:44 ` Tian, Kevin
2009-11-05 8:22 ` Avi Kivity
2009-11-01 11:56 ` [PATCH 03/11] Handle asynchronous page fault in a PV guest Gleb Natapov
2009-11-02 12:38 ` Avi Kivity
2009-11-02 15:54 ` Gleb Natapov
2009-11-03 14:14 ` Marcelo Tosatti
2009-11-03 14:25 ` Gleb Natapov
2009-11-03 14:32 ` Marcelo Tosatti
2009-11-03 14:38 ` Avi Kivity
2009-11-01 11:56 ` [PATCH 04/11] Export __get_user_pages_fast Gleb Natapov
2009-11-02 9:23 ` Ingo Molnar
2009-11-01 11:56 ` [PATCH 05/11] Add get_user_pages() variant that fails if major fault is required Gleb Natapov
2009-11-02 19:05 ` Rik van Riel
2009-11-01 11:56 ` [PATCH 06/11] Inject asynchronous page fault into a guest if page is swapped out Gleb Natapov
2009-11-02 12:56 ` Avi Kivity
2009-11-02 15:41 ` Gleb Natapov
2009-11-01 11:56 ` [PATCH 07/11] Retry fault before vmentry Gleb Natapov
2009-11-02 13:03 ` Avi Kivity
2009-11-01 11:56 ` [PATCH 08/11] Add "wait for page" hypercall Gleb Natapov
2009-11-02 13:05 ` Avi Kivity
2009-11-02 15:13 ` Gleb Natapov
2009-11-02 15:19 ` Avi Kivity
2009-11-01 11:56 ` [PATCH 09/11] Maintain preemptability count even for !CONFIG_PREEMPT kernels Gleb Natapov
2009-11-02 9:24 ` Ingo Molnar
2009-11-01 11:56 ` [PATCH 10/11] Handle async PF in non preemptable context Gleb Natapov
2009-11-01 11:56 ` [PATCH 11/11] Send async PF when guest is not in userspace too Gleb Natapov
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