On Thu, 31 Aug 2023 at 20:01, Colton Lewis <coltonlewis@xxxxxxxxxx> wrote: > > Due to recent KVM changes, QEMU is setting a ptimer offset resulting > in unintended trap and emulate access and a consequent performance > hit. Filter out the PTIMER_CNT register to restore trapless ptimer > access. > > Quoting Andrew Jones: > > Simply reading the CNT register and writing back the same value is > enough to set an offset, since the timer will have certainly moved > past whatever value was read by the time it's written. QEMU > frequently saves and restores all registers in the get-reg-list array, > unless they've been explicitly filtered out (with Linux commit > 680232a94c12, KVM_REG_ARM_PTIMER_CNT is now in the array). So, to > restore trapless ptimer accesses, we need a QEMU patch to filter out > the register. > > See > https://lore.kernel.org/kvmarm/gsntttsonus5.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/T/#m0770023762a821db2a3f0dd0a7dc6aa54e0d0da9 > for additional context. > > Signed-off-by: Andrew Jones <andrew.jones@xxxxxxxxx> > --- Applied to target-arm.next, thanks. -- PMM
