https://bugzilla.kernel.org/show_bug.cgi?id=201753 --- Comment #19 from David Coe (david.coe@xxxxxxxxxx) --- Hi all! It's a knotty problem: the Ryzen has IOMMU performance counters, but can't tell you they're working without a (120 msec 2400g, > 200 msec 2200G) delay (not popular with Paul or Alex). It can assume they are working pro-tem but check later in the boot sequence (Alex's patch but Suravee has some products that don't like this). It can assume they aren't working (although they are) and leave the user-space application to handle the loss of facility (not at all keen on this myself). System OEM's will want clean boot interfaces and road-warriors with high-end laptops hate unnecessary delay when they lift their lids. However, day-to-day users of virtualisation software like (for example) KDM will notice if IOMMU/Vi hardware acceleration is present but crippled by the OS (how do the Windows drivers handle the same issue?). The usual solution for this sort of dilemma, I think, is to put in a BIOS switch or have a boot command-line option. The current default is to turn slow-reporting IOMMU performance counters off. This gives problems later with: # perf stat -a -e amd_iommu/mem_trans_total/ test event syntax error: 'amd_iommu/mem_trans_total/' \___ Cannot find PMU `amd_iommu'. Missing kernel support? After either Suravee or Alex's patch: # perf stat -a -e amd_iommu/mem_trans_total/ test Performance counter stats for 'system wide': 897 amd_iommu/mem_trans_total/ 0.000990106 seconds time elapsed Installation checks for KVM like systool -m kvm_amd -v and kvm-ok then give expected results about modules and hardware acceleration. In principle, I'm sorted for the 2400G but repeatedly recompiling the kernel is not a lovely solution. Very, very appreciative of all attempts to get a generally-acceptable patch into the distributions! Best regards David -- You may reply to this email to add a comment. You are receiving this mail because: You are watching the assignee of the bug.
