On Sun, Sep 10, 2023 at 04:29:02AM -0400, guoren@xxxxxxxxxx wrote:
> From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
>
> Add a static key controlling whether virt_spin_lock() should be
> called or not. When running on bare metal set the new key to
> false.
>
> The KVM guests fall back to a Test-and-Set spinlock, because fair
> locks have horrible lock 'holder' preemption issues. The
> virt_spin_lock_key would shortcut for the
> queued_spin_lock_slowpath() function that allow virt_spin_lock to
> hijack it.
>
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> ---
> .../admin-guide/kernel-parameters.txt | 4 +++
> arch/riscv/include/asm/sbi.h | 8 +++++
> arch/riscv/include/asm/spinlock.h | 22 ++++++++++++++
> arch/riscv/kernel/sbi.c | 2 +-
> arch/riscv/kernel/setup.c | 30 ++++++++++++++++++-
> 5 files changed, 64 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 61cacb8dfd0e..f75bedc50e00 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3927,6 +3927,10 @@
> no_uaccess_flush
> [PPC] Don't flush the L1-D cache after accessing user data.
>
> + no_virt_spin [RISC-V] Disable virt_spin_lock in KVM guest to use
> + native_queued_spinlock when the nopvspin option is enabled.
> + This would help vcpu=pcpu scenarios.
> +
> novmcoredd [KNL,KDUMP]
> Disable device dump. Device dump allows drivers to
> append dump data to vmcore so you can collect driver
> diff --git a/arch/riscv/include/asm/sbi.h b/arch/riscv/include/asm/sbi.h
> index 501e06e52078..e0233b3d7a5f 100644
> --- a/arch/riscv/include/asm/sbi.h
> +++ b/arch/riscv/include/asm/sbi.h
> @@ -50,6 +50,13 @@ enum sbi_ext_base_fid {
> SBI_EXT_BASE_GET_MIMPID,
> };
>
> +enum sbi_ext_base_impl_id {
> + SBI_EXT_BASE_IMPL_ID_BBL = 0,
> + SBI_EXT_BASE_IMPL_ID_OPENSBI,
> + SBI_EXT_BASE_IMPL_ID_XVISOR,
> + SBI_EXT_BASE_IMPL_ID_KVM,
> +};
> +
> enum sbi_ext_time_fid {
> SBI_EXT_TIME_SET_TIMER = 0,
> };
> @@ -269,6 +276,7 @@ int sbi_console_getchar(void);
> long sbi_get_mvendorid(void);
> long sbi_get_marchid(void);
> long sbi_get_mimpid(void);
> +long sbi_get_firmware_id(void);
> void sbi_set_timer(uint64_t stime_value);
> void sbi_shutdown(void);
> void sbi_send_ipi(unsigned int cpu);
> diff --git a/arch/riscv/include/asm/spinlock.h b/arch/riscv/include/asm/spinlock.h
> index 8ea0fee80652..6b38d6616f14 100644
> --- a/arch/riscv/include/asm/spinlock.h
> +++ b/arch/riscv/include/asm/spinlock.h
> @@ -4,6 +4,28 @@
> #define __ASM_RISCV_SPINLOCK_H
>
> #ifdef CONFIG_QUEUED_SPINLOCKS
> +/*
> + * The KVM guests fall back to a Test-and-Set spinlock, because fair locks
> + * have horrible lock 'holder' preemption issues. The virt_spin_lock_key
> + * would shortcut for the queued_spin_lock_slowpath() function that allow
> + * virt_spin_lock to hijack it.
> + */
> +DECLARE_STATIC_KEY_TRUE(virt_spin_lock_key);
> +
> +#define virt_spin_lock virt_spin_lock
> +static inline bool virt_spin_lock(struct qspinlock *lock)
> +{
> + if (!static_branch_likely(&virt_spin_lock_key))
> + return false;
> +
> + do {
> + while (atomic_read(&lock->val) != 0)
> + cpu_relax();
> + } while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0);
> +
> + return true;
> +}
> +
> #define _Q_PENDING_LOOPS (1 << 9)
> #endif
>
> diff --git a/arch/riscv/kernel/sbi.c b/arch/riscv/kernel/sbi.c
> index 88eea3a99ee0..cdd45edc8db4 100644
> --- a/arch/riscv/kernel/sbi.c
> +++ b/arch/riscv/kernel/sbi.c
> @@ -555,7 +555,7 @@ static inline long sbi_get_spec_version(void)
> return __sbi_base_ecall(SBI_EXT_BASE_GET_SPEC_VERSION);
> }
>
> -static inline long sbi_get_firmware_id(void)
> +long sbi_get_firmware_id(void)
> {
> return __sbi_base_ecall(SBI_EXT_BASE_GET_IMP_ID);
> }
> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> index 0f084f037651..c57d15b05160 100644
> --- a/arch/riscv/kernel/setup.c
> +++ b/arch/riscv/kernel/setup.c
> @@ -26,6 +26,7 @@
> #include <asm/alternative.h>
> #include <asm/cacheflush.h>
> #include <asm/cpu_ops.h>
> +#include <asm/cpufeature.h>
> #include <asm/early_ioremap.h>
> #include <asm/pgtable.h>
> #include <asm/setup.h>
> @@ -283,16 +284,43 @@ DEFINE_STATIC_KEY_TRUE(combo_qspinlock_key);
> EXPORT_SYMBOL(combo_qspinlock_key);
> #endif
>
> +#ifdef CONFIG_QUEUED_SPINLOCKS
> +static bool no_virt_spin_key = false;
I suggest no _key, also there is no need for "= false".
To be consistent with enable_qspinlock, I also suggest
adding __ro_after_init:
static bool no_virt_spin __ro_after_init;
> +DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);
> +
> +static int __init no_virt_spin_setup(char *p)
> +{
> + no_virt_spin_key = true;
> +
> + return 0;
> +}
> +early_param("no_virt_spin", no_virt_spin_setup);
> +
> +static void __init virt_spin_lock_init(void)
> +{
> + if (sbi_get_firmware_id() != SBI_EXT_BASE_IMPL_ID_KVM ||
> + no_virt_spin_key)
> + static_branch_disable(&virt_spin_lock_key);
> + else
> + pr_info("Enable virt_spin_lock\n");
> +}
> +#endif
> +
A new virt_no_spin kernel parameter was introduced, but without
CONFIG_QUEUED_SPINLOCKS it will silently fail.
I would suggest an #else clause here with a function to print an error /
warning message about no_virt_spin being invalid in this scenario.
It will probably help future debugging.
> static void __init riscv_spinlock_init(void)
> {
> #ifdef CONFIG_RISCV_COMBO_SPINLOCKS
> - if (!enable_qspinlock_key) {
> + if (!enable_qspinlock_key &&
> + (sbi_get_firmware_id() != SBI_EXT_BASE_IMPL_ID_KVM)) {
> static_branch_disable(&combo_qspinlock_key);
> pr_info("Ticket spinlock: enabled\n");
> } else {
> pr_info("Queued spinlock: enabled\n");
> }
> #endif
> +
> +#ifdef CONFIG_QUEUED_SPINLOCKS
> + virt_spin_lock_init();
> +#endif
> }
>
> extern void __init init_rt_signal_env(void);
> --
> 2.36.1
>
I am probably missing something out, but it looks to me that this patch is
causing 2 different changes:
1 - Enabling no_virt_spin parameter
2 - Disabling queued spinlocks for some firmware_id
Wouldn't be better to split those changes in multiple patches?
Or am I missing the point on why they need to be together?
Thanks!
Leo
