Re: [Xen-devel] managing address space inside paravirtualized guest

[Mike Rapoport <mike.rapoport@xxxxxxxxx>]

Hi Ian,
Thanks a lot for your help.It seems that I've solved the problem after
fixing bugs in the page table pinning code.

--
Sincerely yours,
Mike.

On Tue, Aug 16, 2011 at 6:31 PM, Ian Campbell
<Ian.Campbell@xxxxxxxxxxxxx> wrote:
> On Tue, 2011-08-16 at 14:44 +0100, Mike Rapoport wrote:
>> On Mon, Aug 15, 2011 at 12:33 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> 
>> wrote:
>> > On Sun, 2011-08-14 at 06:59 +0100, Mike Rapoport wrote:
>> >> Hello all,
>> >>
>> >> I working on a project that runs in paravirtualized Linux. The
>> >> application should have it's own address space that is not managed by
>> >> the underlying Linux kernel. I use a kernel module to allocate pages
>> >> for the application page table and to communicate the pages physical
>> >> and machine physical addresses between the kernel and the application.
>> >> The page tables I create in the application seem to be correct and I
>> >> can successfully pin them using Xen hypercalls.
>> >
>> > What is your end goal here?
>>
>> Unfortunately I cannot elaborate about the application because of NDA, but I
>> can tell that certain  parts of the application are required to have
>> control over
>> hardware MMU and interrupts.
>
> :-/ and the requirement is for the _application_ to control these
> mappings etc rather than simply asking the kernel to do it (i.e. by
> mmap'ing device)? Have you seen drivers/uio in the kernel for example?
>
> Anyway, perhaps you could post a dummy user application (which just
> creates the PT and perhaps flip to/from them?), along with the complete
> hypervisor and Linux modifications? I don't think anyone is going to be
> able to help if they are guessing what you might have actually done.
>
>> > Does this scheme work for you under native Linux?
>>
>> Yes, it does.
>>
>> >  In general doing an
>> > end-run around the OS like this seems likely to be fraught with
>> > pitfalls.
>>
>> Agree :)
>>
>> >>  However, when I try to
>> >> set cr3 to point to these page tables with MMUEXT_NEW_{USER}BASEPTR I
>> >> get the following error:
>> >>
>> >> (XEN) domain_crash_sync called from entry.S
>> >> (XEN) Domain 1 (vcpu#0) crashed on cpu#0:
>> >> (XEN) ----[ Xen-4.0.1  x86_64  debug=n  Not tainted ]----
>> >> (XEN) CPU:    0
>> >> (XEN) RIP:    e033:[<0000000fb0013d09>]
>> >
>> > What does this address correspond to?
>>
>> This addres corresponds to the printf("success") in the following code:
>>
>> {
>>     struct mmuext_op op;
>>     int success_count;
>>     int ret;
>>
>>     op.cmd = MMUEXT_NEW_BASEPTR;
>>     op.arg1.mfn = new_cr3 >> PAGE_SHIFT;
>>
>>     ret = HYPERVISOR_mmuext_op(&op, 1, &success_count, DOMID_SELF);
>>     if (ret || success_count != 1)
>>         printf("%s: ret=%d, success_count=%d\n", __func__, ret, 
>> success_count);
>>
>>     printf("%s: success\n", __func__);
>> }
>>
>> i.e. the hypercall is apparently returned succesfully, but the further
>> execution faults.
>
> Where does new_cr3 come from?
>
> Are you sure that your new pagetables include mappings for the kernel
> text and data etc?
>
> There was a cr2 value in the trace, I wonder if it is valid at this
> point (it's not clear if you've taken a page fault or some other form of
> fault)
>
>> >> Any leads how to debug it would be highly appreciated.
>> >
>> > There's only a few calls to domain_crash_sync in entry.S and they all
>> > involve errors while creating a bounce frame (i.e. setting up a return
>> > to guest context with an event injection).
>> >
>> > Since you are replacing cr3 you are presumably taking steps to ensure no
>> > interrupts or anything like that can occur, since they will necessarily
>> > want to be running on the kernel's page tables and not some other
>> > application controlled pagetables.
>>
>> We have the interrupts disabled. Besides, the behavior is consistent and I
>> wouldn't expect that if the reason for faults were interrupts...
>
> OK.
>

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