On Wed, Jun 05, 2024 at 04:20:34PM -0700, Sean Christopherson wrote: > On Fri, 08 Mar 2024 17:09:24 -0800, Sean Christopherson wrote: > > First, rip out KVM's support for virtualizing guest MTRRs on VMX. The > > code is costly to main, a drag on guest boot performance, imperfect, and > > not required for functional correctness with modern guest kernels. Many > > details in patch 1's changelog. > > > > With MTRR virtualization gone, always honor guest PAT on Intel CPUs that > > support self-snoop, as such CPUs are guaranteed to maintain coherency > > even if the guest is aliasing memtypes, e.g. if the host is using WB but > > the guest is using WC. Honoring guest PAT is desirable for use cases > > where the guest must use WC when accessing memory that is DMA'd from a > > non-coherent device that does NOT bounce through VFIO, e.g. for mediated > > virtual GPUs. > > > > [...] > > Applied to kvm-x86 mtrrs, to get as much testing as possible before a potential > merge in 6.11. > > Paul, if you can take a gander at patch 3, it would be much appreciated. > > Thanks! > > [1/5] KVM: x86: Remove VMX support for virtualizing guest MTRR memtypes > https://github.com/kvm-x86/linux/commit/0a7b73559b39 > [2/5] KVM: VMX: Drop support for forcing UC memory when guest CR0.CD=1 > https://github.com/kvm-x86/linux/commit/e1548088ff54 > [3/5] srcu: Add an API for a memory barrier after SRCU read lock > https://github.com/kvm-x86/linux/commit/fcfe671e0879 Looks straightforward enough. We could combine this with the existing smp_mb__after_srcu_read_unlock(), but if we did that, someone would no doubt come up with some clever optimization that provided a full barrier in srcu_read_lock() but not in srcu_read_unlock() or vice versa. So: Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > [4/5] KVM: x86: Ensure a full memory barrier is emitted in the VM-Exit path > https://github.com/kvm-x86/linux/commit/eb8d8fc29286 > [5/5] KVM: VMX: Always honor guest PAT on CPUs that support self-snoop > https://github.com/kvm-x86/linux/commit/95200f24b862 > > -- > https://github.com/kvm-x86/linux/tree/next
