Re: [PATCH kernel 1/3] x86/amd/dr_addr_mask: Cache values in percpu variables

[Sean Christopherson <seanjc@google.com>]

On Thu, Dec 01, 2022, Alexey Kardashevskiy wrote:
> Reading DR[0-3]_ADDR_MASK MSRs takes about 250 cycles which is going to
> be noticeable when the AMD KVM SEV-ES's DebugSwap feature is enabled and
> KVM needs to store these before switching to a guest; the DebugSwitch
> hardware support restores them as type B swap.
>
> This stores MSR values from set_dr_addr_mask() in percpu values and
> returns them via new get_dr_addr_mask(). The gain here is about 10x.
>
> Signed-off-by: Alexey Kardashevskiy <aik@amd.com>
> ---
>  arch/x86/include/asm/debugreg.h |  1 +
>  arch/x86/kernel/cpu/amd.c       | 32 ++++++++++++++++++++
>  2 files changed, 33 insertions(+)
> 
> diff --git a/arch/x86/include/asm/debugreg.h b/arch/x86/include/asm/debugreg.h
> index cfdf307ddc01..c4324d0205b5 100644
> --- a/arch/x86/include/asm/debugreg.h
> +++ b/arch/x86/include/asm/debugreg.h
> @@ -127,6 +127,7 @@ static __always_inline void local_db_restore(unsigned long dr7)
>  
>  #ifdef CONFIG_CPU_SUP_AMD
>  extern void set_dr_addr_mask(unsigned long mask, int dr);
> +extern unsigned long get_dr_addr_mask(int dr);
>  #else
>  static inline void set_dr_addr_mask(unsigned long mask, int dr) { }

KVM_AMD doesn't depend on CPU_SUP_AMD, i.e. this needs a stub.  Or we need to add
a dependency.

> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index c75d75b9f11a..ec7efcef4e14 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -1158,6 +1158,11 @@ static bool cpu_has_amd_erratum(struct cpuinfo_x86 *cpu, const int *erratum)
>  	return false;
>  }
>  
> +DEFINE_PER_CPU_READ_MOSTLY(unsigned long, dr0_addr_mask);
> +DEFINE_PER_CPU_READ_MOSTLY(unsigned long, dr1_addr_mask);
> +DEFINE_PER_CPU_READ_MOSTLY(unsigned long, dr2_addr_mask);
> +DEFINE_PER_CPU_READ_MOSTLY(unsigned long, dr3_addr_mask);
> +
>  void set_dr_addr_mask(unsigned long mask, int dr)
>  {
>  	if (!boot_cpu_has(X86_FEATURE_BPEXT))
> @@ -1166,17 +1171,44 @@ void set_dr_addr_mask(unsigned long mask, int dr)
>  	switch (dr) {
>  	case 0:
>  		wrmsr(MSR_F16H_DR0_ADDR_MASK, mask, 0);

LOL, I'd love to hear how MSR_F16H_DR0_ADDR_MASK ended up with a completely
different MSR index.

> +		per_cpu(dr0_addr_mask, smp_processor_id()) = mask;

Use an array to avoid the copy+paste?  And if you're going to add a cache, might
as well use it to avoid unnecessary writes.

>  		break;
>  	case 1: