Some questions about processing page fault in kvm

Some questions about processing page fault in kvm

[Gu, Zhongshu]

Hi all:
        I have some confusion when looking at the code to process page
fault. If there is vm exit caused by page fault, what is the timing of
this vm exit? In my understanding that it will vm exit before entering
the page fault handler in the kernel. But when it enters vm again,
will it enter the guest kernel's page fault handler again?
       And another problem is that prefetch_page in paging_tmpl.h is
used to optimize kvm to reduce the number of vm exit. Is there any
possibility to bypass the guest page fault? I mean all the page faults
are delegated to kvm to process, no matter it is the page fault of
guest page table or shadow page table.

Thanks

-- 
Zhongshu

Re: Some questions about processing page fault in kvm

[Avi Kivity]

  On 07/23/2010 06:02 PM, Gu, Zhongshu wrote:
> Hi all:
>          I have some confusion when looking at the code to process page
> fault. If there is vm exit caused by page fault, what is the timing of
> this vm exit? In my understanding that it will vm exit before entering
> the page fault handler in the kernel.

Yes.  Some page faults are delivered directly to the guest, though.  
These are all guest faults with ept or npt enabled, and some not-present 
faults on Intel in shadow mode.

>   But when it enters vm again,
> will it enter the guest kernel's page fault handler again?

What do you mean "again"?

It will enter the guest fault handler if it is a guest fault.

>         And another problem is that prefetch_page in paging_tmpl.h is
> used to optimize kvm to reduce the number of vm exit. Is there any
> possibility to bypass the guest page fault? I mean all the page faults
> are delegated to kvm to process, no matter it is the page fault of
> guest page table or shadow page table.

Not all.  See shadow_notrap_nonpresent_pte.

-- 
error compiling committee.c: too many arguments to function

Re: Some questions about processing page fault in kvm

[Gu, Zhongshu]

On Sun, Jul 25, 2010 at 4:52 AM, Avi Kivity <avi@redhat.com> wrote:
>  On 07/23/2010 06:02 PM, Gu, Zhongshu wrote:
>>
>> Hi all:
>>         I have some confusion when looking at the code to process page
>> fault. If there is vm exit caused by page fault, what is the timing of
>> this vm exit? In my understanding that it will vm exit before entering
>> the page fault handler in the kernel.
>
> Yes.  Some page faults are delivered directly to the guest, though.  These
> are all guest faults with ept or npt enabled, and some not-present faults on
> Intel in shadow mode.
So if i set the module parameter of bypass_guest_pf to 0, all the page
faults will cause vm exit. Is that right?

>
>>  But when it enters vm again,
>> will it enter the guest kernel's page fault handler again?
>
> What do you mean "again"?
>
> It will enter the guest fault handler if it is a guest fault.
if there is guest fault, it means that the gpte is non-present. If the
bypass_guest_pf is set to 1, there will be no possibility for this
situation.
>
>>        And another problem is that prefetch_page in paging_tmpl.h is
>> used to optimize kvm to reduce the number of vm exit. Is there any
>> possibility to bypass the guest page fault? I mean all the page faults
>> are delegated to kvm to process, no matter it is the page fault of
>> guest page table or shadow page table.
>
> Not all.  See shadow_notrap_nonpresent_pte.
>

> --
> error compiling committee.c: too many arguments to function
>
>



-- 
Zhongshu

Re: Some questions about processing page fault in kvm

[Avi Kivity]

  On 07/25/2010 07:31 PM, Gu, Zhongshu wrote:
>
>> Yes.  Some page faults are delivered directly to the guest, though.  These
>> are all guest faults with ept or npt enabled, and some not-present faults on
>> Intel in shadow mode.
> So if i set the module parameter of bypass_guest_pf to 0, all the page
> faults will cause vm exit. Is that right?

It is (in addition, make sure ept=0).

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

Virtualization technology conferences/tutorials/courses to attend?Wednesday, August 4, 2010 6:59 PM

[Anjali Kulkarni]

Hi,

I am new to Virtualization, so can someone point me to 
conferences/tutorials/courses that I can attend related to Linux KVM,
para-virtualization, device driver virtualization, hardware assisted
virtualization etc.? Even documents pointers will be much useful.

Thanks!
Anjali

Virtualization technology conferences/tutorials/courses to attend?

[Anjali Kulkarni]

Hi,

I am new to Virtualization, so can someone point me to 
conferences/tutorials/courses that I can attend related to Linux KVM,
para-virtualization, device driver virtualization, hardware assisted
virtualization etc.? Even documents pointers will be much useful.

Thanks!
Anjali

Re: Virtualization technology conferences/tutorials/courses to attend?

[Anthony Liguori]

On 08/04/2010 06:17 PM, Anjali Kulkarni wrote:
> Hi,
>
> I am new to Virtualization, so can someone point me to 
> conferences/tutorials/courses that I can attend related to Linux KVM,
> para-virtualization, device driver virtualization, hardware assisted
> virtualization etc.? Even documents pointers will be much useful.
>    

KVM Forum is next week.  See http://www.linux-kvm.org/page/KVM_Forum_2010

There will also be a virtualization track at Plumbers in November.  For 
plan documentation, the KVM and QEMU wikis would be a good start.

Regards,

Anthony Liguori

> Thanks!
> Anjali
>
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