Re: [PATCH v2 2/2] amd_iommu: Support 64 bit address for IOTLB lookup

[Sairaj Kodilkar <sarunkod@amd.com>]

On 10/13/2025 1:49 PM, Michael S. Tsirkin wrote:
> On Mon, Oct 13, 2025 at 10:30:46AM +0530, Sairaj Kodilkar wrote:
>> Physical AMD IOMMU supports up to 64 bits of DMA address. When device tries
>> to read or write from a given DMA address, IOMMU translates the address
>> using page table assigned to that device. Since IOMMU uses per device page
>> tables, the emulated IOMMU should use the cache tag of 68 bits
>> (64 bit address - 12 bit page alignment + 16 bit device ID).
>>
>> Current emulated AMD IOMMU uses GLib hash table to create software iotlb
>> and uses 64 bit key to store the IOVA and deviceID, which limits the IOVA
>> to 60 bits. This causes failure while setting up the device when guest is
>> booted with "iommu.forcedac=1".
>>
>> To solve this problem, Use 64 bit IOVA and 16 bit devid as key to store
>> entries in IOTLB; Use upper 52 bits of IOVA (GFN) and lower 12 bits of
>> the device ID to construct the 64 bit hash key in order avoid the
>> truncation as much as possible (reducing hash collisions).
>>
>> Fixes: d29a09ca6842 ("hw/i386: Introduce AMD IOMMU")
>> Signed-off-by: Sairaj Kodilkar<sarunkod@amd.com>
> I am wondering whether we need to limit how much host memory
> can the shadow take. Because with so many bits, the sky is the limit ...
> OTOH it's not directly caused by this patch, but it's something
> we should think about maybe.

I don't think I fully understand this. Do you mean the host memory
taken by the pages used to build shadow page table ?


> Something more to improve:
>
>
>> ---
>>   hw/i386/amd_iommu.c | 57 ++++++++++++++++++++++++++++++---------------
>>   hw/i386/amd_iommu.h |  4 ++--
>>   2 files changed, 40 insertions(+), 21 deletions(-)
>>
>> diff --git a/hw/i386/amd_iommu.c b/hw/i386/amd_iommu.c
>> index b194e3294dd7..a218d147e53d 100644
>> --- a/hw/i386/amd_iommu.c
>> +++ b/hw/i386/amd_iommu.c
>> @@ -106,6 +106,11 @@ typedef struct amdvi_as_key {
>>       uint8_t devfn;
>>   } amdvi_as_key;
>>   
>> +typedef struct amdvi_iotlb_key {
>> +    uint64_t gfn;
>> +    uint16_t devid;
>> +} amdvi_iotlb_key;
>> +
> Pls change struct and typedef names to match qemu coding style.
Thanks
Sairaj