[Xen-devel] Re: Linux Stubdom Problem

[Jiageng Yu <yujiageng734@xxxxxxxxx>]

2011/7/27 Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>:
> On Tue, 26 Jul 2011, Jiageng Yu wrote:
>> 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,
>>
>> Â Â 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
>> Â Â Â->qemu_get_ram_ptr(s->vram_offset)
>> Â Â Â Â Â Â->block->host = xen_map_block(block->offset, block->length);
>> Â Â Â Â Â Â Â Â Â->xc_map_foreign_bulk()
>> Â Â Â Â Â Â Â Â Â Â Â Â ->linux_privcmd_map_foreign_bulk()
>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â->xen_remap_domain_mfn_range()
>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â->HYPERVISOR_mmu_update()
>> __________________________________________________________
>>
>> Xen
>> __________________________________________________________
>> do_mmu_update()
>> Â Â->case MMU_PT_UPDATE_PRESERVE_AD:
>> Â Â->case PGT_l1_page_table:
>> Â Â->mod_l1_entry(va, l1e, mfn,cmd == MMU_PT_UPDATE_PRESERVE_AD, v, 
>> pg_owner);
>> Â Â Â Â Â ->mfn_x(gfn_to_mfn(p2m_get_hostp2m(pg_dom),
>> l1e_get_pfn(nl1e), &p2mt));
>> Â Â Â Â Â Â Â Â Â->gfn_to_mfn_type_p2m()
>> Â Â Â Â Â Â Â Â Â Â Â Â ->p2m->get_entry(p2m, gfn, t, &a, q);
>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â->p2m_gfn_to_mfn(p2m,gfn,t,&a,q)
>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â ->if ( (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 ÂError 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?
>
>
> It seems that you are trying to map pages that don't exist.
> The pages in question should be allocated by:
>
> qemu_ram_alloc(NULL, "vga.vram", vga_ram_size)
> Â Âqemu_ram_alloc_from_ptr
> Â Â Â Âxen_ram_alloc
> Â Â Â Â Â Âxc_domain_populate_physmap_exact
>
> so I would add some printf and printk on this code path to find out if
> xc_domain_populate_physmap_exact fails for some reasons.

Hmm.. the linux stubdom kernel had a wrong p2m pair
<gfn(0x40000),mfn(0x127bd2)> for some reason. But next, the
xc_domain_populate_physmap_exact will setup the correct p2m pair
<gfn(0x40000),mfn(0x896b7)>. However, the p2m pair in stubdom kernel
has not been updated, because the fllowing access to 0x40000 still
uses 0x127bd2.

I notice you have a patch: xen: modify kernel mappings corresponding
to granted pages. I think maybe it could slove my problem.

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