Re: [Xen-devel] [Patch RFC 00/13] VT-d Asynchronous Device-TLB Flush for ATS Device

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.09.15 at 15:55, <quan.xu@xxxxxxxxx> wrote:
>> >> >> >>> On September 29, 2015 at 3:22 PM, <JBeulich@xxxxxxxx> wrote:
>> >>> On 29.09.15 at 04:53, <quan.xu@xxxxxxxxx> wrote:
>> >>>> Monday, September 28, 2015 2:47 PM,<JBeulich@xxxxxxxx> wrote:
>> >> >>> On 28.09.15 at 05:08, <quan.xu@xxxxxxxxx> wrote:
>> >> >>>> Thursday, September 24, 2015 12:27 AM, Tim Deegan wrote:
>> >
>> > For Tim's suggestion --"to make the IOMMU table take typed refcounts
>> > to anything it points to, and only drop those refcounts when the flush
>> > completes."
>> >
>> > From IOMMU point of view, if it can walk through IOMMU table to get
>> > these pages and take typed refcounts.
>> > These pages are maybe owned by hardware_domain, dummy, HVM guest .etc.
>> > could I narrow it down to HVM guest? --- It is not for anything it
>> > points to, but just for HVM guest related. this will simplify the design.
>> 
>> I don't follow. Why would you want to walk page tables? And why would a HVM
>> guest have pages other than those owned by itself or granted access to by
>> another guest mapped in its IOMMU page tables?
> 
> It is tricky. Let's ignore it.
> 
> This is an analysis of IOMMU table to take typed refcounts to anything it 
> points to.
> I know the IOMMU table and EPT table may share the same page 
> table('iommu_hap_pt_share = 1').
> Then, go through iommu table and take typed refcounts.
> 
>> In any event - the ref-counting
>> would need to happen as you _create_ the mappings, not at some later point.
>> 
> a general rule. Agreed.
> 
> When create the mappings, what conditions to take typed refcounts?

On any r/o mapping, take a general ref. On any r/w mapping take
a writable ref.

>> > --- a/xen/include/asm-x86/mm.h
>> > +++ b/xen/include/asm-x86/mm.h
>> > @@ -183,6 +183,7 @@ struct page_info
>> >  #define PGT_seg_desc_page PG_mask(5, 4)  /* using this page in a GDT/LDT?
>> */
>> >  #define PGT_writable_page PG_mask(7, 4)  /* has writable mappings?
>> */
>> >  #define PGT_shared_page   PG_mask(8, 4)  /* CoW sharable page
>> */
>> > +#define PGT_dev_tlb_page  PG_mask(9, 4)  /* Maybe in Device-TLB
>> mapping?   */
>> >  #define PGT_type_mask     PG_mask(15, 4) /* Bits 28-31 or 60-63.
>> */
>> >
>> > * I define a new typed refcounts PGT_dev_tlb_page.
>> 
>> Why? I.e. why won't a base ref for r/o pages and a writable type-ref for r/w 
> ones
>> suffice, just like we do everywhere else?
>> 
> 
> I think it is different from r/o or writable.
> 
> The page freed from the domain, the Device-TLB flush is not completed.
> The page is _not_ r/o or writable, and can only access though DMA..

DMA or not, the page can either be accessed (read) (when mapped
r/o in the IOMMU) or written (when mapped r/w in the IOMMU). For
the purpose of refcounting it doesn't matter where the reads/writes
originate.

Jan


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