• Dave Martin's avatar
    KVM: arm64: Optimise FPSIMD handling to reduce guest/host thrashing · e6b673b7
    Dave Martin authored
    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: default avatarDave Martin <Dave.Martin@arm.com>
    Reviewed-by: default avatarMarc Zyngier <marc.zyngier@arm.com>
    Reviewed-by: default avatarChristoffer Dall <christoffer.dall@arm.com>
    Reviewed-by: default avatarAlex Bennée <alex.bennee@linaro.org>
    Acked-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
    Signed-off-by: default avatarMarc Zyngier <marc.zyngier@arm.com>
    e6b673b7
kvm_host.h 15.8 KB