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Re: [Xen-devel] [PATCH 1/6] x86/vtd: Don't include control register state in the table pointers


  • To: George Dunlap <dunlapg@xxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Thu, 28 Feb 2019 11:28:25 +0000
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  • Cc: Kevin Tian <kevin.tian@xxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>
  • Delivery-date: Thu, 28 Feb 2019 11:28:45 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 25/02/2019 11:39, George Dunlap wrote:
> On Fri, Feb 22, 2019 at 7:15 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
> wrote:
>> iremap_maddr and qinval_maddr point to the base of a block of contiguous RAM,
>> allocated by the driver, holding the Interrupt Remapping table, and the 
>> Queued
>> Invalidation ring.
>>
>> Despite their name, they are actually the values of the hardware register,
>> including control metadata in the lower 12 bits.  While uses of these fields
>> do appear to correctly shift out the metadata, this is very subtle behaviour
>> and confusing to follow.
> Could I suggest that we add an ASSERT() to that macro, that maddr is
> page aligned?
>
> The algorithm used certainly assumes that's the case, and would
> produce incorrect results if it were ever validly true.  Such an
> ASSERT() would also have detected the storing-garbage-in-maddr issue
> almost immediately.

Where would you suggest putting such an ASSERT() ?  This variable is
written exactly once.

Irrespective, I plan to replace the existing scheme with a vmap()'d one
which will delete this variable completely.

~Andrew

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