Hi, I found that in the guest Linux OS. The IVSHMEM device is set with 3 MTRR regions.
reg00: base=0x0c0000000 ( 3072MB), size= 1024MB, count=1: uncachable reg01: base=0x0b0000000 ( 2816MB), size= 256MB, count=1: uncachable reg02: base=0x800000000 (32768MB), size=32768MB, count=1: uncachable Bar 2 is listed in reg02. As the entry said, it's set to uncachable. This entry causes slow access on AMD machine even though I map the region to be writeback using devm_memmap_pages with type set to MEMORY_DEVICE_GENERIC. By removing the entry, the access to the region goes faster as it's cacheable. Inside the kernel driver, there's no way to remove those entries as the mtrr_del are not exposed. I have to do it in user space. Does anyone know what sets these MTRR entries? By default, the virtio-pci driver is used for the IVSHMEM device. I saw the device driver use ioremap to map the bar. But what I saw in the source code is it sets the PAT not MTRR. Best, Zhiting
