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

[Alexey Kardashevskiy]

On 2/12/22 03:58, Sean Christopherson wrote:
> 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@xxxxxxx>
> > ---
> >   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.

The new symbol is under CONFIG_CPU_SUP_AMD and so is its only user 
arch/x86/kernel/cpu/amd.c:

arch/x86/kernel/cpu/Makefile:
obj-$(CONFIG_CPU_SUP_AMD)               += amd.o


Is this enough dependency or we need something else? (if enough, I'll 
put it into the next commit log).


> > 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.

I'll do this, I did not realize DEFINE_PER_CPU_READ_MOSTLY can do 
arrays. Thanks,

> >   		break;
> >   	case 1:

--
Alexey