Re: [PATCH 1/2] IOMMU/MMU: Adjust top level functions for VT-d Device-TLB flush error.

From: George Dunlap <George.Dunlap@eu.citrix.com>
To: Quan Xu <quan.xu@intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>, Keir Fraser <keir@xen.org>,
	Jun Nakajima <jun.nakajima@intel.com>,
	Liu Jinsong <jinsong.liu@alibaba-inc.com>,
	Dario Faggioli <dario.faggioli@citrix.com>,
	"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>,
	Jan Beulich <jbeulich@suse.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Feng Wu <feng.wu@intel.com>
Subject: Re: [PATCH 1/2] IOMMU/MMU: Adjust top level functions for VT-d Device-TLB flush error.
Date: Thu, 17 Mar 2016 12:33:38 +0000	[thread overview]
Message-ID: <CAFLBxZZ9=vbcKQJS=NvjdNf+1uvOzSDOkAg8FNLDv3CC2ygXfw@mail.gmail.com> (raw)
In-Reply-To: <CAFLBxZaTyOE61YX+qe_2qXU=zF32fr-409LK5OSm4QzT+z1EJw@mail.gmail.com>

On Thu, Mar 17, 2016 at 12:30 PM, George Dunlap
<George.Dunlap@eu.citrix.com> wrote:
> On Thu, Mar 17, 2016 at 6:54 AM, Quan Xu <quan.xu@intel.com> wrote:
>> diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
>> index c997b53..526548e 100644
>> --- a/xen/arch/x86/mm.c
>> +++ b/xen/arch/x86/mm.c
>> @@ -2467,7 +2467,7 @@ static int __get_page_type(struct page_info *page, unsigned long type,
>>                             int preemptible)
>>  {
>>      unsigned long nx, x, y = page->u.inuse.type_info;
>> -    int rc = 0;
>> +    int rc = 0, ret = 0;
>>
>>      ASSERT(!(type & ~(PGT_type_mask | PGT_pae_xen_l2)));
>>
>> @@ -2578,11 +2578,11 @@ static int __get_page_type(struct page_info *page, unsigned long type,
>>          if ( d && is_pv_domain(d) && unlikely(need_iommu(d)) )
>>          {
>>              if ( (x & PGT_type_mask) == PGT_writable_page )
>> -                iommu_unmap_page(d, mfn_to_gmfn(d, page_to_mfn(page)));
>> +                ret = iommu_unmap_page(d, mfn_to_gmfn(d, page_to_mfn(page)));
>>              else if ( type == PGT_writable_page )
>> -                iommu_map_page(d, mfn_to_gmfn(d, page_to_mfn(page)),
>> -                               page_to_mfn(page),
>> -                               IOMMUF_readable|IOMMUF_writable);
>> +                ret = iommu_map_page(d, mfn_to_gmfn(d, page_to_mfn(page)),
>> +                                     page_to_mfn(page),
>> +                                     IOMMUF_readable|IOMMUF_writable);
>>          }
>>      }
>>
>> @@ -2599,6 +2599,9 @@ static int __get_page_type(struct page_info *page, unsigned long type,
>>      if ( (x & PGT_partial) && !(nx & PGT_partial) )
>>          put_page(page);
>>
>> +    if ( !rc )
>> +        rc = ret;
>> +
>
> What's this about?  If the iommu_[un]map_page() operation times out,
> we still go through with calling alloc_page_type(); and if
> alloc_page_type() fails we return its failure value, but if it
> succeeds, we return the error from iommu_[un]map_page()?
>
>>      return rc;
>>  }
>>
>> diff --git a/xen/arch/x86/mm/p2m-ept.c b/xen/arch/x86/mm/p2m-ept.c
>> index 3cb6868..f9bcce7 100644
>> --- a/xen/arch/x86/mm/p2m-ept.c
>> +++ b/xen/arch/x86/mm/p2m-ept.c
>> @@ -830,7 +830,15 @@ out:
>>          {
>>              if ( iommu_flags )
>>                  for ( i = 0; i < (1 << order); i++ )
>> -                    iommu_map_page(d, gfn + i, mfn_x(mfn) + i, iommu_flags);
>> +                {
>> +                    rc = iommu_map_page(d, gfn + i, mfn_x(mfn) + i, iommu_flags);
>> +                    if ( rc )
>> +                    {
>> +                        while ( i-- > 0 )
>> +                            iommu_unmap_page(d, gfn + i);
>
> This won't unmap gfn+0 (since it will break out when i == 0 without
> calling unmap).

Oh, no it won't, because the decrement is postfix.

For us mere mortals, I'd appreciate a comment here like this:

/* Postfix operator means we will call unmap with i == 0 */

Thanks,
 -George

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