Re: [Xen-devel] RFC: AMD support for paging

["Andres Lagar-Cavilla" <andres@xxxxxxxxxxxxxxxx>]

> On 02/14/2012 08:05 PM, Andres Lagar-Cavilla wrote:
>> We started hashing out some AMD support for mem_paging and mem_access.
>> Right now my VMs boot, page out a bit, and then die on an HVM triple
>> fault.
>>
>> Most importantly, I want to get somebody from AMD to comment/help out on
>> this. It feels like we're inches away from enabling support for this
>> very
>> nice feature. I'm not sure who exactly on AMD monitors the list for
>> these
>> kinds of things. It'd be great to have you on board!
>>
>> For starters, the changes to the p2m code are relatively mild, but it'd
>> be
>> great if somebody spots a red flag.
>>
>> Another issue: comments indicate that bits 59-62 in NPT entries are used
>> for IOMMU flags but effectively bits 61-62 are. Repossessing one bit
>> (59)
>> would give us enough space to support mem_access. Right now we only have
>> 7
>> bits available for Xen flags and that is not enough for paging and
>> access.
>> Is bit 59 effectively reserved?
>
> Hi
> bit 59 is used by iommu hardware for ATS response. In most cases, for
> p2m_ram_rw pages, U bit must be 0. But maybe for other page types that
> are not potentially used by DMA, this bit could be non-zero. I could
> tested it on my iommu machines if you had some patches that use U bits.

Hi Wei, thanks for pitching in! I assume you're talking about table 14
(and fig 9) in http://support.amd.com/us/Processor_TechDocs/48882.pdf

I don't think this will work. The p2m access value is arbitrary, and
independent of the p2m type. So bit 59 is out of reach and we're stuck
with 7 bits for Xen use. Thanks for the clarification.

An alternative to enabling mem_access on AMD processors will be to limit
the access types to 3 bits. This would force disabling support for two
modes. I'm inclined to disable two out of X, W and WX. I don't think they
make sense without R permissions.

Thanks!
Andres

>
> Thanks,
> Wei
>
>>
>> Finally, the triple fault. Maybe I'm missing something obvious. Comments
>> welcome.
>>
>> Patch inline below, thanks!
>> Andres
>>
>> Enable AMD support for paging.
>>
>> Signed-off-by: Andres Lagar-Cavilla<andres@xxxxxxxxxxxxxxxx>
>> Signed-off-by: Adin Scannell<adin@xxxxxxxxxxx>
>>
>> diff -r 25ca78889ed4 -r 10ca4e4293ce xen/arch/x86/mm/mem_event.c
>> --- a/xen/arch/x86/mm/mem_event.c
>> +++ b/xen/arch/x86/mm/mem_event.c
>> @@ -537,10 +537,6 @@ int mem_event_domctl(struct domain *d, x
>>               if ( !hap_enabled(d) )
>>                   break;
>>
>> -            /* Currently only EPT is supported */
>> -            if ( boot_cpu_data.x86_vendor != X86_VENDOR_INTEL )
>> -                break;
>> -
>>               rc = -EXDEV;
>>               /* Disallow paging in a PoD guest */
>>               if ( p2m->pod.entry_count )
>> diff -r 25ca78889ed4 -r 10ca4e4293ce xen/arch/x86/mm/p2m-pt.c
>> --- a/xen/arch/x86/mm/p2m-pt.c
>> +++ b/xen/arch/x86/mm/p2m-pt.c
>> @@ -53,6 +53,20 @@
>>   #define P2M_BASE_FLAGS \
>>           (_PAGE_PRESENT | _PAGE_USER | _PAGE_DIRTY | _PAGE_ACCESSED)
>>
>> +#ifdef __x86_64__
>> +/* l1e_from_pfn is not designed to have INVALID_MFN stored. The
>> 0xff..ff
>> + * value tramples over the higher-order bits used for flags (NX, p2mt,
>> + * etc.) This happens for paging entries. Thus we do this clip/unclip
>> + * juggle for l1 entries only (no paging superpages!) */
>> +#define EFF_MFN_WIDTH       (PADDR_BITS-PAGE_SHIFT) /* 40 bits */
>> +#define clipped_mfn(mfn)    ((mfn)&  ((1UL<<  EFF_MFN_WIDTH) - 1))
>> +#define unclip_mfn(mfn)     ((clipped_mfn((mfn)) == INVALID_MFN) ? \
>> +                                INVALID_MFN : (mfn))
>> +#else
>> +#define clipped_mfn(mfn)    (mfn)
>> +#define unclip_mfn(mfn)     (mfn)
>> +#endif /* __x86_64__ */
>> +
>>   static unsigned long p2m_type_to_flags(p2m_type_t t, mfn_t mfn)
>>   {
>>       unsigned long flags;
>> @@ -77,6 +91,9 @@ static unsigned long p2m_type_to_flags(p
>>       case p2m_invalid:
>>       case p2m_mmio_dm:
>>       case p2m_populate_on_demand:
>> +    case p2m_ram_paging_out:
>> +    case p2m_ram_paged:
>> +    case p2m_ram_paging_in:
>>       default:
>>           return flags;
>>       case p2m_ram_ro:
>> @@ -168,7 +185,7 @@ p2m_next_level(struct p2m_domain *p2m, m
>>                                         shift, max)) )
>>           return 0;
>>
>> -    /* PoD: Not present doesn't imply empty. */
>> +    /* PoD/paging: Not present doesn't imply empty. */
>>       if ( !l1e_get_flags(*p2m_entry) )
>>       {
>>           struct page_info *pg;
>> @@ -384,8 +401,9 @@ p2m_set_entry(struct p2m_domain *p2m, un
>>                                      0, L1_PAGETABLE_ENTRIES);
>>           ASSERT(p2m_entry);
>>
>> -        if ( mfn_valid(mfn) || (p2mt == p2m_mmio_direct) )
>> -            entry_content = l1e_from_pfn(mfn_x(mfn),
>> +        if ( mfn_valid(mfn) || (p2mt == p2m_mmio_direct) ||
>> +             (p2mt == p2m_ram_paged) || (p2mt == p2m_ram_paging_in) )
>> +            entry_content = l1e_from_pfn(clipped_mfn(mfn_x(mfn)),
>>                                            p2m_type_to_flags(p2mt,
>> mfn));
>>           else
>>               entry_content = l1e_empty();
>> @@ -393,7 +411,7 @@ p2m_set_entry(struct p2m_domain *p2m, un
>>           if ( entry_content.l1 != 0 )
>>           {
>>               p2m_add_iommu_flags(&entry_content, 0, iommu_pte_flags);
>> -            old_mfn = l1e_get_pfn(*p2m_entry);
>> +            old_mfn = unclip_mfn(l1e_get_pfn(*p2m_entry));
>>           }
>>           /* level 1 entry */
>>           p2m->write_p2m_entry(p2m, gfn, p2m_entry, table_mfn,
>> entry_content, 1);
>> @@ -615,11 +633,12 @@ pod_retry_l1:
>>                              sizeof(l1e));
>>
>>       if ( ret == 0 ) {
>> +        unsigned long l1e_mfn = unclip_mfn(l1e_get_pfn(l1e));
>>           p2mt = p2m_flags_to_type(l1e_get_flags(l1e));
>> -        ASSERT(l1e_get_pfn(l1e) != INVALID_MFN || !p2m_is_ram(p2mt));
>> +        ASSERT( (l1e_mfn != INVALID_MFN || !p2m_is_ram(p2mt)) ||
>> +                (l1e_mfn == INVALID_MFN&&  p2m_is_paging(p2mt)) );
>>
>> -        if ( p2m_flags_to_type(l1e_get_flags(l1e))
>> -             == p2m_populate_on_demand )
>> +        if ( p2mt == p2m_populate_on_demand )
>>           {
>>               /* The read has succeeded, so we know that the mapping
>>                * exits at this point.  */
>> @@ -641,7 +660,7 @@ pod_retry_l1:
>>           }
>>
>>           if ( p2m_is_valid(p2mt) || p2m_is_grant(p2mt) )
>> -            mfn = _mfn(l1e_get_pfn(l1e));
>> +            mfn = _mfn(l1e_mfn);
>>           else
>>               /* XXX see above */
>>               p2mt = p2m_mmio_dm;
>> @@ -783,18 +802,26 @@ pod_retry_l2:
>>   pod_retry_l1:
>>       if ( (l1e_get_flags(*l1e)&  _PAGE_PRESENT) == 0 )
>>       {
>> +        p2m_type_t l1t = p2m_flags_to_type(l1e_get_flags(*l1e));
>>           /* PoD: Try to populate */
>> -        if ( p2m_flags_to_type(l1e_get_flags(*l1e)) ==
>> p2m_populate_on_demand )
>> +        if ( l1t == p2m_populate_on_demand )
>>           {
>>               if ( q != p2m_query ) {
>>                   if ( !p2m_pod_demand_populate(p2m, gfn, PAGE_ORDER_4K,
>> q) )
>>                       goto pod_retry_l1;
>>               } else
>>                   *t = p2m_populate_on_demand;
>> +        } else {
>> +            if ( p2m_is_paging(l1t) )
>> +            {
>> +                *t = l1t;
>> +                /* No need to unclip due to check below */
>> +                mfn = _mfn(l1e_get_pfn(*l1e));
>> +            }
>>           }
>>
>>           unmap_domain_page(l1e);
>> -        return _mfn(INVALID_MFN);
>> +        return (l1t == p2m_ram_paging_out) ? mfn : _mfn(INVALID_MFN);
>>       }
>>       mfn = _mfn(l1e_get_pfn(*l1e));
>>       *t = p2m_flags_to_type(l1e_get_flags(*l1e));
>> @@ -914,7 +941,7 @@ static void p2m_change_type_global(struc
>>                       flags = l1e_get_flags(l1e[i1]);
>>                       if ( p2m_flags_to_type(flags) != ot )
>>                           continue;
>> -                    mfn = l1e_get_pfn(l1e[i1]);
>> +                    mfn = unclip_mfn(l1e_get_pfn(l1e[i1]));
>>                       gfn = i1 + (i2 + (i3
>>   #if CONFIG_PAGING_LEVELS>= 4
>>                                      + (i4 * L3_PAGETABLE_ENTRIES)
>> @@ -923,7 +950,7 @@ static void p2m_change_type_global(struc
>>                              * L2_PAGETABLE_ENTRIES) *
>> L1_PAGETABLE_ENTRIES;
>>                       /* create a new 1le entry with the new type */
>>                       flags = p2m_type_to_flags(nt, _mfn(mfn));
>> -                    l1e_content = l1e_from_pfn(mfn, flags);
>> +                    l1e_content = l1e_from_pfn(clipped_mfn(mfn),
>> flags);
>>                       p2m->write_p2m_entry(p2m, gfn,&l1e[i1],
>>                                            l1mfn, l1e_content, 1);
>>                   }
>> @@ -1073,7 +1100,7 @@ long p2m_pt_audit_p2m(struct p2m_domain
>>                                   entry_count++;
>>                               continue;
>>                           }
>> -                        mfn = l1e_get_pfn(l1e[i1]);
>> +                        mfn = unclip_mfn(l1e_get_pfn(l1e[i1]));
>>                           ASSERT(mfn_valid(_mfn(mfn)));
>>                           m2pfn = get_gpfn_from_mfn(mfn);
>>                           if ( m2pfn != gfn&&
>>
>>
>> _______________________________________________
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>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>> http://lists.xensource.com/xen-devel
>>
>
>
>



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