On 09/05/18 17:12, Dave Martin wrote:
> This patch refactors KVM to align the host and guest FPSIMD
> save/restore logic with each other for arm64. This reduces the
> number of redundant save/restore operations that must occur, and
> reduces the common-case IRQ blackout time during guest exit storms
> by saving the host state lazily and optimising away the need to
> restore the host state before returning to the run loop.
>
> Four hooks are defined in order to enable this:
>
> * kvm_arch_vcpu_run_map_fp():
> Called on PID change to map necessary bits of current to Hyp.
>
> * kvm_arch_vcpu_load_fp():
> Set up FP/SIMD for entering the KVM run loop (parse as
> "vcpu_load fp").
>
> * kvm_arch_vcpu_ctxsync_fp():
> Get FP/SIMD into a safe state for re-enabling interrupts after a
> guest exit back to the run loop.
>
> For arm64 specifically, this involves updating the host kernel's
> FPSIMD context tracking metadata so that kernel-mode NEON use
> will cause the vcpu's FPSIMD state to be saved back correctly
> into the vcpu struct. This must be done before re-enabling
> interrupts because kernel-mode NEON may be used by softirqs.
>
> * kvm_arch_vcpu_put_fp():
> Save guest FP/SIMD state back to memory and dissociate from the
> CPU ("vcpu_put fp").
>
> Also, the arm64 FPSIMD context switch code is updated to enable it
> to save back FPSIMD state for a vcpu, not just current. A few
> helpers drive this:
>
> * fpsimd_bind_state_to_cpu(struct user_fpsimd_state *fp):
> mark this CPU as having context fp (which may belong to a vcpu)
> currently loaded in its registers. This is the non-task
> equivalent of the static function fpsimd_bind_to_cpu() in
> fpsimd.c.
>
> * task_fpsimd_save():
> exported to allow KVM to save the guest's FPSIMD state back to
> memory on exit from the run loop.
>
> * fpsimd_flush_state():
> invalidate any context's FPSIMD state that is currently loaded.
> Used to disassociate the vcpu from the CPU regs on run loop exit.
>
> These changes allow the run loop to enable interrupts (and thus
> softirqs that may use kernel-mode NEON) without having to save the
> guest's FPSIMD state eagerly.
>
> Some new vcpu_arch fields are added to make all this work. Because
> host FPSIMD state can now be saved back directly into current's
> thread_struct as appropriate, host_cpu_context is no longer used
> for preserving the FPSIMD state. However, it is still needed for
> preserving other things such as the host's system registers. To
> avoid ABI churn, the redundant storage space in host_cpu_context is
> not removed for now.
>
> arch/arm is not addressed by this patch and continues to use its
> current save/restore logic. It could provide implementations of
> the helpers later if desired.
>
> Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>
>
> ---
>
> Dropped Reviewed-bys due to non-trivial changes.
>
> Changes since v6:
>
> * Don't define kvm_arch_vcpu_run_pid_change() unless CONFIG_KVM=y.
>
> <asm/kvm_host.h> may be used for its declarations even with
> CONFIG_KVM=n (e.g., in asm-offsets.c).
>
> This patch avoids conflicts with the core headers in this config.
>
> * Rebind current's FP state to the cpu in vcpu_put() if it is
> still loaded, to ensure that the SVE trapping setup for userspace is
> properly restored.
>
> Requested by Marc Zyngier:
>
> * Add a comment to explain the purpose of update_fp_enabled().
>
> * Migrate vcpu_arch.flags definitions to kvm_host.h.
>
> * Eliminate magic NULL semantics for vcpu_arch.host_fpsimd_state so
> that we can just assign this pointer once in the pid_change hook.
>
> A new flag KVM_ARM64_FP_HOST flag is added to capture the former
> semantics of vcpu->arch.host_fpsimd_state != NULL.
Thanks for the additional rework, that looks much better now.
Reviewed-by: Marc Zyngier <marc.zyngier@xxxxxxx>
M.
--
Jazz is not dead. It just smells funny...
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