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

[Sairaj Kodilkar]

On 10/14/2025 2:35 PM, Michael S. Tsirkin wrote:

On Tue, Oct 14, 2025 at 02:34:28PM +0530, Sairaj Kodilkar wrote:

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<sarun...@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 ?

the memory used by the hash table.

Right now it defines macro 'AMDVI_IOTLB_MAX_SIZE'. If iotlb hash
size is greater than that, code resets the hash table.

Techinically, right way is to implement a policy such as LRU, which
we can work on in future patches.

Thanks
Sairaj

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