Re: [PATCH v3] x86/paravirt: Disable virt spinlock on bare metal

[Nikolay Borisov <nik.borisov@suse.com>]

On 25.06.24 г. 15:54 ч., Chen Yu wrote:
> The kernel can change spinlock behavior when running as a guest. But
> this guest-friendly behavior causes performance problems on bare metal.
> So there's a 'virt_spin_lock_key' static key to switch between the two
> modes.
> 
> The static key is always enabled by default (run in guest mode) and
> should be disabled for bare metal (and in some guests that want native
> behavior).
> 
> Performance drop is reported when running encode/decode workload and
> BenchSEE cache sub-workload.
> Bisect points to commit ce0a1b608bfc ("x86/paravirt: Silence unused
> native_pv_lock_init() function warning"). When CONFIG_PARAVIRT_SPINLOCKS
> is disabled the virt_spin_lock_key is incorrectly set to true on bare
> metal. The qspinlock degenerates to test-and-set spinlock, which
> decrease the performance on bare metal.
> 
> Set the default value of virt_spin_lock_key to false. If booting in a VM,
> enable this key. Later during the VM initialization, if other
> high-efficient spinlock is preferred(paravirt-spinlock eg), the
> virt_spin_lock_key is disabled accordingly. The relation is described as
> below:
> 
> X86_FEATURE_HYPERVISOR         Y    Y    Y     N
> CONFIG_PARAVIRT_SPINLOCKS      Y    Y    N     Y/N
> PV spinlock                    Y    N    N     Y/N
> 
> virt_spin_lock_key             N    N    Y     N
> 
> Fixes: ce0a1b608bfc ("x86/paravirt: Silence unused native_pv_lock_init() function warning")
> Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> Suggested-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
> Suggested-by: Nikolay Borisov <nik.borisov@suse.com>
> Reported-by: Prem Nath Dey <prem.nath.dey@intel.com>
> Reported-by: Xiaoping Zhou <xiaoping.zhou@intel.com>
> Signed-off-by: Chen Yu <yu.c.chen@intel.com>
> ---
> v2._v3:
>    Change the default value of virt_spin_lock_key from true to false.
>    Enable this key when it is in the VM, and disable it when needed.
>    This makes the code more readable. (Nikolay Borisov)
>    Dropped Reviewed-by because the code has been changed.
> v1->v2:
>    Refine the commit log per Dave's suggestion.
>    Simplify the fix by directly disabling the virt_spin_lock_key on bare metal.
>    Collect Reviewed-by from Juergen.
> ---
>   arch/x86/include/asm/qspinlock.h | 4 ++--
>   arch/x86/kernel/paravirt.c       | 7 +++----
>   2 files changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
> index a053c1293975..a32bd2aabdf9 100644
> --- a/arch/x86/include/asm/qspinlock.h
> +++ b/arch/x86/include/asm/qspinlock.h
> @@ -66,13 +66,13 @@ static inline bool vcpu_is_preempted(long cpu)
>   
>   #ifdef CONFIG_PARAVIRT
>   /*
> - * virt_spin_lock_key - enables (by default) the virt_spin_lock() hijack.
> + * virt_spin_lock_key - disables (by default) the virt_spin_lock() hijack.
>    *
>    * Native (and PV wanting native due to vCPU pinning) should disable this key.
>    * It is done in this backwards fashion to only have a single direction change,
>    * which removes ordering between native_pv_spin_init() and HV setup.
>    */
> -DECLARE_STATIC_KEY_TRUE(virt_spin_lock_key);
> +DECLARE_STATIC_KEY_FALSE(virt_spin_lock_key);
>   
>   /*
>    * Shortcut for the queued_spin_lock_slowpath() function that allows
> diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
> index 5358d43886ad..fec381533555 100644
> --- a/arch/x86/kernel/paravirt.c
> +++ b/arch/x86/kernel/paravirt.c
> @@ -51,13 +51,12 @@ DEFINE_ASM_FUNC(pv_native_irq_enable, "sti", .noinstr.text);
>   DEFINE_ASM_FUNC(pv_native_read_cr2, "mov %cr2, %rax", .noinstr.text);
>   #endif
>   
> -DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);
> +DEFINE_STATIC_KEY_FALSE(virt_spin_lock_key);
>   
>   void __init native_pv_lock_init(void)
>   {
> -	if (IS_ENABLED(CONFIG_PARAVIRT_SPINLOCKS) &&

Actually now shouldn't the CONFIG_PARAVIRT_SPINLOCKS check be retained? 
Otherwise we'll have the virtspinlock enabled even if we are a guest but 
CONFIG_PARAVIRT_SPINLOCKS is disabled, no ?

> -	    !boot_cpu_has(X86_FEATURE_HYPERVISOR))
> -		static_branch_disable(&virt_spin_lock_key);
> +	if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
> +		static_branch_enable(&virt_spin_lock_key);
>   }
>   
>   static void native_tlb_remove_table(struct mmu_gather *tlb, void *table)