[Xen-devel] Re: Linux Stubdom Problem

[Jiageng Yu <yujiageng734@xxxxxxxxx>]

2011/7/26 Jiageng Yu <yujiageng734@xxxxxxxxx>:
> 2011/7/22 Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>:
>> On Thu, 21 Jul 2011, Jiageng Yu wrote:
>>> 2011/7/19 Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>:
>>> > CC'ing Tim and xen-devel
>>> >
>>> > On Mon, 18 Jul 2011, Jiageng Yu wrote:
>>> >> 2011/7/16 Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>:
>>> >> > On Fri, 15 Jul 2011, Jiageng Yu wrote:
>>> >> >> 2011/7/15 Jiageng Yu <yujiageng734@xxxxxxxxx>:
>>> >> >> > 2011/7/15 Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>:
>>> >> >> >> On Fri, 15 Jul 2011, Jiageng Yu wrote:
>>> >> >> >>> > Does it mean you are actually able to boot an HVM guest using 
>>> >> >> >>> > Linux
>>> >> >> >>> > based stubdoms?? Did you manage to solve the framebuffer 
>>> >> >> >>> > problem too?
>>> >> >> >>>
>>> >> >> >>>
>>> >> >> >>> The HVM guest is booted. But the boot process is terminated 
>>> >> >> >>> because
>>> >> >> >>> vga bios is not invoked by seabios. I have got stuck here for a 
>>> >> >> >>> week.
>>> >> >> >>>
>>> >> >> >>
>>> >> >> >> There was a bug in xen-unstable.hg or seabios that would prevent 
>>> >> >> >> vga bios from
>>> >> >> >> being loaded, it should be fixed now.
>>> >> >> >>
>>> >> >> >> Alternatively you can temporarely work around the issue with this 
>>> >> >> >> hacky patch:
>>> >> >> >>
>>> >> >> >> ---
>>> >> >> >>
>>> >> >> >>
>>> >> >> >> diff -r 00d2c5ca26fd tools/firmware/hvmloader/hvmloader.c
>>> >> >> >> --- a/tools/firmware/hvmloader/hvmloader.c   ÂFri Jul 08 
>>> >> >> >> 18:35:24 2011 +0100
>>> >> >> >> +++ b/tools/firmware/hvmloader/hvmloader.c   ÂFri Jul 15 
>>> >> >> >> 11:37:12 2011 +0000
>>> >> >> >> @@ -430,7 +430,7 @@ int main(void)
>>> >> >> >> Â Â Â Â Â Â bios->create_pir_tables();
>>> >> >> >> Â Â }
>>> >> >> >>
>>> >> >> >> - Â Âif ( bios->load_roms )
>>> >> >> >> + Â Âif ( 1 )
>>> >> >> >> Â Â {
>>> >> >> >> Â Â Â Â switch ( virtual_vga )
>>> >> >> >> Â Â Â Â {
>>> >> >> >>
>>> >> >> >>
>>> >> >> >
>>> >> >> > Yes. Vga bios is booted. However, the upstram qemu receives a 
>>> >> >> > SIGSEGV
>>> >> >> > signal subsequently. I am trying to print the call stack when
>>> >> >> > receiving the signal.
>>> >> >> >
>>> >> >>
>>> >> >> Hi,
>>> >> >>
>>> >> >> Â ÂI find the cause of SIGSEGV signal:
>>> >> >>
>>> >> >> Â Âcpu_physical_memory_rw(target_phys_addr_t addr, uint8_t *buf, int
>>> >> >> len, int is_write)
>>> >> >> Â Â Â Â Â Â Â Â Â ->memcpy(buf, ptr + (addr & ~TARGET_PAGE_MASK), l);
>>> >> >>
>>> >> >> Â Â In my case, ptr=0 and addr=0xc253e, when qemu attempts to vist
>>> >> >> 0x53e address, the SIGSEGV signal is generated.
>>> >> >>
>>> >> >> Â Â I believe the qemu is trying to vist vram in this moment. This
>>> >> >> code seems no problem, and I will continue to find the root cause.
>>> >> >>
>>> >> >
>>> >> > The vram is allocated by qemu, see hw/vga.c:vga_common_init.
>>> >> > qemu_ram_alloc under xen ends up calling xen_ram_alloc that calls
>>> >> > xc_domain_populate_physmap_exact.
>>> >> > xc_domain_populate_physmap_exact is the hypercall that should ask Xen 
>>> >> > to
>>> >> > add the missing vram pages in the guest. Maybe this hypercall is 
>>> >> > failing
>>> >> > in your case?
>>> >>
>>> >>
>>> >> Hi,
>>> >>
>>> >> Â ÂI continue to invesgate this bug and find hypercall_mmu_update in
>>> >> qemu_remap_bucket(xc_map_foreign_bulk) is failing:
>>> >>
>>> >> do_mmu_update
>>> >> Â Â Â ->mod_l1_entry
>>> >> Â Â Â Â Â Â Â-> Âif ( !p2m_is_ram(p2mt) || unlikely(mfn == INVALID_MFN) )
>>> >> Â Â Â Â Â Â Â Â Â Â Â Â Âreturn -EINVAL;
>>> >>
>>> >> Â Âmfn==INVALID_MFN, because :
>>> >>
>>> >> mod_l1_entry
>>> >> Â Â Â ->gfn_to_mfn(p2m_get_hostp2m(pg_dom), l1e_get_pfn(nl1e), &p2mt));
>>> >> Â Â Â Â Â Â Â ->p2m->get_entry
>>> >> Â Â Â Â Â Â Â Â Â Â Â Â ->p2m_gfn_to_mfn
>>> >> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â-> if ( gfn > p2m->max_mapped_pfn )
>>> >> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â/* This pfn is higher than the
>>> >> highest the p2m map currently holds */
>>> >> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Âreturn _mfn(INVALID_MFN);
>>> >>
>>> >> Â ÂThe p2m->max_mapped_pfn is usually 0xfffff. In our case,
>>> >> mmu_update.val exceeds 0x8000000100000000. ÂAdditionally, l1e =
>>> >> l1e_from_intpte(mmu_update.val); gfn=l1e_get_pfn(l1e ). Therefore, gfn
>>> >> will exceed 0xfffff.
>>> >>
>>> >> Â ÂIn the case of minios based stubdom, the mmu_update.vals do not
>>> >> exceed 0x8000000100000000. Next, I will invesgate why mmu_update.val
>>> >> exceeds 0x8000000100000000.
>>> >
>>> > It looks like the address of the guest that qemu is trying to map is not
>>> > valid.
>>> > Make sure you are running a guest with less than 2GB of ram, otherwise
>>> > you need the patch series that Anthony sent on Friday:
>>> >
>>> > http://marc.info/?l=qemu-devel&m=131074042905711&w=2
>>>
>>> Not this problem. I never alloc more than 2GB for the hvm guest. The
>>> call stack in qemu is:
>>>
>>> qemu_get_ram_ptr
>>> Â Â Â ->qemu_map_cache(addr, 0, 1)
>>> Â Â Â Â Â Â Â Â Â-> if (!entry->vaddr_base || entry->paddr_index !=
>>> address_index ||
>>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â !test_bit(address_offset >>
>>> XC_PAGE_SHIFT, entry->valid_mapping)) {
>>> Â Â Â Â Â Â Â Â Â Â Â Â Â Âqemu_remap_bucket(entry, size ? :
>>> MCACHE_BUCKET_SIZE, address_index);
>>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â->xc_map_foreign_bulk(xen_xc,
>>> xen_domid, PROT_READ|PROT_WRITE,
>>>
>>> Â Â Â Â Â Â Â Â pfns, err, nb_pfn);
>>>
>>> The qemu tries to map pages from hvm guest(xen_domid) to linux
>>> stubdom. But some hvm pages' pfns are larger than 0xfffff. So, in the
>>> p2m_gfn_to_mfn, the judgement condition is valid:(p2m->max_mapped_pfn
>>> = 0xfffff)
>>>
>>> Â Â if ( gfn > p2m->max_mapped_pfn )
>>> Â Â Â Â /* This pfn is higher than the highest the p2m map currently holds 
>>> */
>>> Â Â Â Â return _mfn(INVALID_MFN);
>>>
>>> ÂIn minios stubdom case, the hvm pages' pfns do not exceed 0xfffff.
>>> Maybe the address translation in linux stubdom cause this probem?
>>
>> Trying to map a pfn > 0xfffff is clearly a mistake if the guest's memory
>> does not exceed 2G:
>>
>> 0xfffff * 4096 > 2G
>>
>>
>>> ÂBTW, in minios stubdom case, there seems no hvmloader process. Is it
>>> needed in linux stubdom?
>>
>> hvmloader is the first thing that runs within the guest, it is not a
>> process in the stubdom or in dom0.
>> It is required in both minios and linux stubdoms.
>
> Hi Stefano,
>
> Â Â ÂI patched these patches, but we still have the same problem.
> However, I noticed the qemu_get_ram_ptr(s->vram_offset) in
> vga_common_init function was also failed. Maybe this can explain the
> previous problem, which happened in the phase of trying to remap
> 0xc0000-0xc8fff of hvm guest into stubdom.
>
> Â Â I have traced the process of qemu_get_ram_ptr(s->vram_offset) and
> located the failure in p2m_gfn_to_mfn function:
>
> Â Â pod_retry_l3:
> Â Â Â Âif ( (l3e_get_flags(*l3e) & _PAGE_PRESENT) == 0 )
> Â Â Â Â{
> Â Â Â Â Â Â Â Â .....
> Â Â Â Â Â Â Â Â return _mfn(INVALID_MFN);
> Â Â Â Â}
>
> Â Â I will continue to analyze this failure.
>
> Â Â Thanks!
>
> Jiageng Yu.
>


Hi,

    I compared the two executions of vga_common_init function in dom0
and linux based stubdom. The former succeeded and the later was
failed. First, they have the same call stack:

Dom0 & Stubdom
_________________________________________________________
vga_common_init
     ->qemu_get_ram_ptr(s->vram_offset)
           ->block->host = xen_map_block(block->offset, block->length);
                 ->xc_map_foreign_bulk()
                        ->linux_privcmd_map_foreign_bulk()
                               ->xen_remap_domain_mfn_range()
                                     ->HYPERVISOR_mmu_update()
__________________________________________________________

Xen
__________________________________________________________
do_mmu_update()
   ->case MMU_PT_UPDATE_PRESERVE_AD:
   ->case PGT_l1_page_table:
   ->mod_l1_entry(va, l1e, mfn,cmd == MMU_PT_UPDATE_PRESERVE_AD, v, pg_owner);
          ->mfn_x(gfn_to_mfn(p2m_get_hostp2m(pg_dom),
l1e_get_pfn(nl1e), &p2mt));
                 ->gfn_to_mfn_type_p2m()
                        ->p2m->get_entry(p2m, gfn, t, &a, q);
                               ->p2m_gfn_to_mfn(p2m,gfn,t,&a,q)
                                      ->if ( (l3e_get_flags(*l3e) &
_PAGE_PRESENT) == 0 )
                                      ->    Error happens!

The qemu in dom0 can find the l3e of hvm guest, but the qemu in linux
stubdom cannot find the l3e. In my case, s->vram_offset=0x40000000,
vga_ram_size=0x800000. Therefore, we are going to map hvm guest's
address area(pfn:0x40000, size:8M) into linux stubdom's address space.

In p2m_gfn_to_mfn function, p2m->domain->domain_id=hvm guest,
gfn=0x40000, t=p2m_mmio_dm.
mfn = pagetable_get_mfn(p2m_get_pagetable(p2m) = 0x10746e;
map_domain_page(mfn_x(mfn)) is also success. However, after executing:
l3e += ( (0x40000 << PAGE_SHIFT) >> L3_PAGETABLE_SHIFT)
the l3e->l3 =0 , and the error happens.

So, in linux stubdom, when we are going to map the specified hvm
guest's address(pfn:0x40000, size:8M), we find these pages of hvm
guest are not present. This is never happened in qemu of dom0. Could
you give me some prompts to this problem?

Thanksï

Jiageng Yu.

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