Re: [Xen-devel] [PATCH 02/11] VMX: implement suppress #VE.

[Ed White <edmund.h.white@xxxxxxxxx>]

On 01/12/2015 08:43 AM, Andrew Cooper wrote:
> On 09/01/15 21:26, Ed White wrote:
>> In preparation for selectively enabling hardware #VE in a later patch,
>> set suppress #VE on all EPTE's on #VE-capable hardware.
>>
>> Suppress #VE should always be the default condition for two reasons:
>> it is generally not safe to deliver #VE into a guest unless that guest
>> has been modified to receive it; and even then for most EPT violations only
>> the hypervisor is able to handle the violation.
>>
>> Signed-off-by: Ed White <edmund.h.white@xxxxxxxxx>
>> ---
>>  xen/arch/x86/mm/p2m-ept.c         | 34 +++++++++++++++++++++++++++++++++-
>>  xen/include/asm-x86/hvm/vmx/vmx.h |  1 +
>>  2 files changed, 34 insertions(+), 1 deletion(-)
>>
>> diff --git a/xen/arch/x86/mm/p2m-ept.c b/xen/arch/x86/mm/p2m-ept.c
>> index eb8b5f9..2b9f07c 100644
>> --- a/xen/arch/x86/mm/p2m-ept.c
>> +++ b/xen/arch/x86/mm/p2m-ept.c
>> @@ -41,7 +41,7 @@
>>  #define is_epte_superpage(ept_entry)    ((ept_entry)->sp)
>>  static inline bool_t is_epte_valid(ept_entry_t *e)
>>  {
>> -    return (e->epte != 0 && e->sa_p2mt != p2m_invalid);
>> +    return (e->valid != 0 && e->sa_p2mt != p2m_invalid);
>>  }
>>  
>>  /* returns : 0 for success, -errno otherwise */
>> @@ -194,6 +194,19 @@ static int ept_set_middle_entry(struct p2m_domain *p2m, 
>> ept_entry_t *ept_entry)
>>  
>>      ept_entry->r = ept_entry->w = ept_entry->x = 1;
>>  
>> +    /* Disable #VE on all entries */ 
>> +    if ( cpu_has_vmx_virt_exceptions )
>> +    {
>> +        ept_entry_t *table = __map_domain_page(pg);
>> +
>> +        for ( int i = 0; i < EPT_PAGETABLE_ENTRIES; i++ )
> 
> Style - please declare i in the upper scope, and it should be unsigned.
> 
>> +            table[i].suppress_ve = 1;
>> +
>> +        unmap_domain_page(table);
>> +
>> +        ept_entry->suppress_ve = 1;
>> +    }
>> +
>>      return 1;
>>  }
>>  
>> @@ -243,6 +256,10 @@ static int ept_split_super_page(struct p2m_domain *p2m, 
>> ept_entry_t *ept_entry,
>>          epte->sp = (level > 1);
>>          epte->mfn += i * trunk;
>>          epte->snp = (iommu_enabled && iommu_snoop);
>> +
>> +        if ( cpu_has_vmx_virt_exceptions )
>> +            epte->suppress_ve = 1;
>> +
>>          ASSERT(!epte->rsvd1);
>>  
>>          ept_p2m_type_to_flags(epte, epte->sa_p2mt, epte->access);
>> @@ -753,6 +770,9 @@ ept_set_entry(struct p2m_domain *p2m, unsigned long gfn, 
>> mfn_t mfn,
>>          ept_p2m_type_to_flags(&new_entry, p2mt, p2ma);
>>      }
>>  
>> +    if ( cpu_has_vmx_virt_exceptions )
>> +        new_entry.suppress_ve = 1;
>> +
>>      rc = atomic_write_ept_entry(ept_entry, new_entry, target);
>>      if ( unlikely(rc) )
>>          old_entry.epte = 0;
>> @@ -1069,6 +1089,18 @@ int ept_p2m_init(struct p2m_domain *p2m)
>>      /* set EPT page-walk length, now it's actual walk length - 1, i.e. 3 */
>>      ept->ept_wl = 3;
>>  
>> +    /* Disable #VE on all entries */
>> +    if ( cpu_has_vmx_virt_exceptions )
>> +    {
>> +        ept_entry_t *table =
>> +            map_domain_page(pagetable_get_pfn(p2m_get_pagetable(p2m)));
>> +
>> +        for ( int i = 0; i < EPT_PAGETABLE_ENTRIES; i++ )
>> +            table[i].suppress_ve = 1;
> 
> Is it safe setting SVE on an entry which is not known to be a superpage
> or not present?  The manual states that the bit is ignored in this case,
> but I am concerned that, as with SVE, this bit will suddenly gain
> meaning in the future.
> 

It is safe to do this. Never say never, but I am aware of no plans to
overload this bit, and I would know. Unless you feel strongly about it,
I would prefer to leave this as-is, since changing it would make the code
more complex.

>> +
>> +        unmap_domain_page(table);
>> +    }
>> +
>>      if ( !zalloc_cpumask_var(&ept->synced_mask) )
>>          return -ENOMEM;
>>  
>> diff --git a/xen/include/asm-x86/hvm/vmx/vmx.h 
>> b/xen/include/asm-x86/hvm/vmx/vmx.h
>> index 8bae195..70fee74 100644
>> --- a/xen/include/asm-x86/hvm/vmx/vmx.h
>> +++ b/xen/include/asm-x86/hvm/vmx/vmx.h
>> @@ -49,6 +49,7 @@ typedef union {
>>          suppress_ve :   1;  /* bit 63 - suppress #VE */
>>      };
>>      u64 epte;
>> +    u64 valid       :   63; /* entire EPTE except suppress #VE bit */
> 
> I am not sure 'valid' is a sensible name here.  As it is only used in
> is_epte_valid(), might it be better to just use ->epte and a bitmask for
> everything other than the #VE bit?
> 

This seemed more in the style of the code I was changing, but I can do it
as you suggest.

Ed

>>  } ept_entry_t;
>>  
>>  typedef struct {
> 
> 

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