Re: [Xen-devel] device-mmio emulation in Xen

[Abhinav Srivastava <abhinavs_iitkgp@xxxxxxxxxxx>]

Hi Tim,

Thanks for the reply. I understood what you had described. However, I have one 
question:

1. How does Xen know which code path to execute? What I am noticing is that 
sometimes it goes to all the way till the end of the page fault handler code to 
call handle_mmio function, and sometimes it goes into the fast path to call 
this function, bypassing all the page fault handlers code? Why it does not go 
into the fast path all the time? OR Xen does something at the first time so 
that second time, the execution can directly go into the fast path? And, does 
this needs to be done for all physical pages given to MMIO?

Thanks,
Abhinav



--- On Tue, 5/1/10, Tim Deegan <Tim.Deegan@xxxxxxxxxx> wrote:

> From: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
> Subject: Re: [Xen-devel] device-mmio emulation in Xen
> To: "Abhinav Srivastava" <abhinavs_iitkgp@xxxxxxxxxxx>
> Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
> Date: Tuesday, 5 January, 2010, 9:30 PM
> Hi, 
> 
> At 15:32 +0000 on 05 Jan (1262705557), Abhinav Srivastava
> wrote:
> > I noticed that during a network copy operation Xen
> page faults a lot and control goes to sh_page_fault
> function. When I printed some debugging info, it showed me
> gmfn = -1. Then the execution goes through the regular path
> of the page fault handler code, which means it creates an
> entry using shadow_get_and_create_l1e, propagates it using
> l1e_propagate_from_guest, and finally updates the entry
> using shadow_set_l1e. It finally goes into the device-model
> mmio condition. In this condition, it extracts a guest
> physical address and calls "goto mmio", which in turn calls
> handle_mmio function that emulates the instruction.
> > 
> > However with the gmfn = -1 condition, the execution
> sometimes directly goto
> > to handle_mmio function using the fast_fault_path with
> going through the regular path. It seems like there are two
> possible execution paths, and I did not understand which one
> is chosen when?
> > 
> 
> /******************************************************************************
>  * We implement a "fast path" for two special cases: faults
> that require
>  * MMIO emulation, and faults where the guest PTE is not
> present.  We
>  * record these as shadow l1 entries that have reserved
> bits set in
>  * them, so we can spot them immediately in the fault
> handler and handle
>  * them without needing to hold the shadow lock or walk the
> guest
>  * pagetables.
>  *
>  * This is only feasible for PAE and 64bit Xen: 32-bit
> non-PAE PTEs don't
>  * have reserved bits that we can use for this.
>  */
> 
> > I have some questions related to this behavior:
> > 
> > 1. Why are there so many faults duing network copy
> operation?
> 
> The faults are a HVM guest doing MMIO operations to control
> its emulated
> network card.
> 
> > 2. What does gmfn = -1 signify? Is it reserved for
> mmio addresses?
> 
> It's INVALID_MFN.  It means there is no memory mapped
> at the address the
> guest accessed.  Xen treats that as MMIO, though
> there's a plan to have
> MMIO regions explicitly registered instead.
> 
> > 3. How does Xen handle this gmfn = -1? It seems like
> on the regular path
> > it still creates, propagates, and updates entries for
> gmfn = -1. How does Xen handles this at the shadow page
> table level?
> 
> Yes, it creates a deliberately invalid shadow PTE; then in
> the pagefault
> handler it can use the pagefault error code and the
> contents of the PTE 
> to skip the bulk of the shadow fault handling and go
> straight to handle_mmio.
> 
> > 4. What are these two code execution paths, and when
> does Xen decide which
> > path to choose?
> > 
> > 5. Finally, is there anyway these faults can be
> reduced?
> 
> Yes, use PV drivers in the guest.  That eliminates the
> whole
> emulated-MMIO system, and all the pagefaults associated
> with it. 
> 
> Cheers,
> 
> Tim.
> 
> > I would very appreciate any help in this regard.
> > 
> > Thanks,
> > Abhinav
> > 
> > 
> > 
> >       The INTERNET now has a
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> > 
> > _______________________________________________
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> 
> -- 
> Tim Deegan <Tim.Deegan@xxxxxxxxxx>
> Principal Software Engineer, Citrix Systems (R&D) Ltd.
> [Company #02300071, SL9 0DZ, UK.]
> 


      The INTERNET now has a personality. YOURS! See your Yahoo! Homepage. 
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