Re: [PATCH v3 1/5] iommu/amd: Disable AMD IOMMU if CMPXCHG16B feature is not supported

[Jason Gunthorpe]

On Fri, Sep 06, 2024 at 01:38:18PM -0300, Jason Gunthorpe wrote:
> On Fri, Sep 06, 2024 at 12:13:04PM +0000, Suravee Suthikulpanit wrote:
> > According to the AMD IOMMU spec, the IOMMU reads the entire DTE either
> > in two 128-bit transactions or a single 256-bit transaction. 
> 
> .. if two 128-bit transaction on the read side is possible then you
> need flushing! :(
> 
> For instance this:
> 
>   IOMMU         CPU
> Read [0]    
>               Write [0]
>               Write [1]
> Read [1]
> 
> Will result in the iommu seeing torn incorrect data - the Guest paging
> mode may not match the page table pointer, or the VIOMMU data may
> become mismatched to the host translation.
> 
> Avoiding flushing is only possible if the full 256 bits are read
> atomically.

Also, please think about what qemu does when paravirtualizing
this. qemu will read the DTE entry using the CPU.

For your above remark it should be reading using two 128 bit loads.

However, it doesn't seem to be doing that:

static bool amdvi_get_dte(AMDVIState *s, int devid, uint64_t *entry)
{
    uint32_t offset = devid * AMDVI_DEVTAB_ENTRY_SIZE;

    if (dma_memory_read(&address_space_memory, s->devtab + offset, entry,
                        AMDVI_DEVTAB_ENTRY_SIZE, MEMTXATTRS_UNSPECIFIED)) {


The dma_memory_read eventually boils down to memcpy()

So qemu looks wrong to me.

Jason