Subject: [PATCH v3 26/41] KVM: arm64: Introduce framework for accessing deferred sysregs

From: [email-masked] (Christoffer Dall)
Date: Thu, 25 Jan 2018 20:54:13 +0100

On Tue, Jan 23, 2018 at 04:04:40PM +0000, Dave Martin wrote:
> On Fri, Jan 12, 2018 at 01:07:32PM +0100, Christoffer Dall wrote:
> > We are about to defer saving and restoring some groups of system
> > registers to vcpu_put and vcpu_load on supported systems.  This means
> > that we need some infrastructure to access system registes which
> > supports either accessing the memory backing of the register or directly
> > accessing the system registers, depending on the state of the system
> > when we access the register.
> > 
> > We do this by defining a set of read/write accessors for each system
> > register, and letting each system register be defined as "immediate" or
> > "deferrable".  Immediate registers are always saved/restored in the
> > world-switch path, but deferrable registers are only saved/restored in
> > vcpu_put/vcpu_load when supported and sysregs_loaded_on_cpu will be set
> > in that case.
> > 
> > Not that we don't use the deferred mechanism yet in this patch, but only
> > introduce infrastructure.  This is to improve convenience of review in
> > the subsequent patches where it is clear which registers become
> > deferred.
> 
> Might this table-driven approach result in a lot of branch mispredicts,
> particularly across load/put boundaries?
> 
> If we were to move the whole construct to a header, then it could get
> constant-folded at the call site down to the individual reg accessed,
> say:
> 
> 	if (sys_regs_loaded)
> 		read_sysreg_s(TPIDR_EL0);
> 	else
> 		__vcpu_sys_reg(v, TPIDR_EL0);
> 
> Where multiple regs are accessed close to each other, the compiler
> may be able to specialise the whole sequence for the loaded and !loaded
> cases so that there is only one conditional branch.
> 

That's an interesting thing to consider indeed.  I wasn't really sure
how to put this in a header file which wouldn't look overly bloated for
inclusion elsewhere, so we ended up with this.

I don't think the alternative suggestion that I discused with Julien on
this patch changes this much, but since you've had a look at this, I'm
curious which one of the two (lookup table vs. giant switch) you prefer?

> 
> The individual accessor functions also become unnecessary in this case,
> because we wouldn't need to derive function pointers from them any
> more.
> 
> I don't know how performance would compare in practice though.

I don't know either.  But I will say that the whole idea behind put/load
is that you do this rarely, and going to userspace from KVM is
notriously expensive, also on x86.

> 
> I'm also assuming that all calls to these accessors are const-foldable.
> If not, relying on inlining would bloat the generated code a lot.

We have places where this is not the cae, access_vm_reg() for example.
But if we really, really, wanted to, we could rewrite that to have a
function for each register, but that's pretty horrid on its own.

Thanks,
-Christoffer

> 
> > 
> >  [ Most of this logic was contributed by Marc Zyngier ]
> > 
> > Signed-off-by: Marc Zyngier <[email-masked]>
> > Signed-off-by: Christoffer Dall <[email-masked]>
> > ---
> >  arch/arm64/include/asm/kvm_host.h |   8 +-
> >  arch/arm64/kvm/sys_regs.c         | 160 ++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 166 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index 91272c35cc36..4b5ef82f6bdb 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -281,6 +281,10 @@ struct kvm_vcpu_arch {
> >  
> >  	/* Detect first run of a vcpu */
> >  	bool has_run_once;
> > +
> > +	/* True when deferrable sysregs are loaded on the physical CPU,
> > +	 * see kvm_vcpu_load_sysregs and kvm_vcpu_put_sysregs. */
> > +	bool sysregs_loaded_on_cpu;
> >  };
> >  
> >  #define vcpu_gp_regs(v)		(&(v)->arch.ctxt.gp_regs)
> > @@ -293,8 +297,8 @@ struct kvm_vcpu_arch {
> >   */
> >  #define __vcpu_sys_reg(v,r)	((v)->arch.ctxt.sys_regs[(r)])
> >  
> > -#define vcpu_read_sys_reg(v,r)	__vcpu_sys_reg(v,r)
> > -#define vcpu_write_sys_reg(v,r,n)	do { __vcpu_sys_reg(v,r) = n; } while (0)
> > +u64 vcpu_read_sys_reg(struct kvm_vcpu *vcpu, int reg);
> > +void vcpu_write_sys_reg(struct kvm_vcpu *vcpu, int reg, u64 val);
> >  
> >  /*
> >   * CP14 and CP15 live in the same array, as they are backed by the
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 96398d53b462..9d353a6a55c9 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -35,6 +35,7 @@
> >  #include <asm/kvm_coproc.h>
> >  #include <asm/kvm_emulate.h>
> >  #include <asm/kvm_host.h>
> > +#include <asm/kvm_hyp.h>
> >  #include <asm/kvm_mmu.h>
> >  #include <asm/perf_event.h>
> >  #include <asm/sysreg.h>
> > @@ -76,6 +77,165 @@ static bool write_to_read_only(struct kvm_vcpu *vcpu,
> >  	return false;
> >  }
> >  
> > +struct sys_reg_accessor {
> > +	u64	(*rdsr)(struct kvm_vcpu *, int);
> > +	void	(*wrsr)(struct kvm_vcpu *, int, u64);
> > +};
> > +
> > +#define DECLARE_IMMEDIATE_SR(i)						\
> > +	static u64 __##i##_read(struct kvm_vcpu *vcpu, int r)		\
> > +	{								\
> > +		return __vcpu_sys_reg(vcpu, r);				\
> > +	}								\
> > +									\
> > +	static void __##i##_write(struct kvm_vcpu *vcpu, int r, u64 v)	\
> > +	{								\
> > +		__vcpu_sys_reg(vcpu, r) = v;				\
> > +	}								\
> > +
> > +#define DECLARE_DEFERRABLE_SR(i, s)					\
> > +	static u64 __##i##_read(struct kvm_vcpu *vcpu, int r)		\
> > +	{								\
> > +		if (vcpu->arch.sysregs_loaded_on_cpu) {			\
> > +			WARN_ON(kvm_arm_get_running_vcpu() != vcpu);	\
> > +			return read_sysreg_s((s));			\
> > +		}							\
> > +		return __vcpu_sys_reg(vcpu, r);				\
> > +	}								\
> > +									\
> > +	static void __##i##_write(struct kvm_vcpu *vcpu, int r, u64 v)	\
> > +	{								\
> > +		if (vcpu->arch.sysregs_loaded_on_cpu) {			\
> > +			WARN_ON(kvm_arm_get_running_vcpu() != vcpu);	\
> > +			write_sysreg_s(v, (s));				\
> > +		} else {						\
> > +			__vcpu_sys_reg(vcpu, r) = v;			\
> > +		}							\
> > +	}								\
> > +
> > +
> > +#define SR_HANDLER_RANGE(i,e)						\
> > +	[i ... e] =  (struct sys_reg_accessor) {			\
> > +		.rdsr = __##i##_read,					\
> > +		.wrsr = __##i##_write,					\
> > +	}
> > +
> > +#define SR_HANDLER(i)	SR_HANDLER_RANGE(i, i)
> > +
> > +static void bad_sys_reg(int reg)
> > +{
> > +	WARN_ONCE(1, "Bad system register access %d\n", reg);
> > +}
> > +
> > +static u64 __default_read_sys_reg(struct kvm_vcpu *vcpu, int reg)
> > +{
> > +	bad_sys_reg(reg);
> > +	return 0;
> > +}
> > +
> > +static void __default_write_sys_reg(struct kvm_vcpu *vcpu, int reg, u64 val)
> > +{
> > +	bad_sys_reg(reg);
> > +}
> > +
> > +/* Ordered as in enum vcpu_sysreg */
> > +DECLARE_IMMEDIATE_SR(MPIDR_EL1);
> > +DECLARE_IMMEDIATE_SR(CSSELR_EL1);
> > +DECLARE_IMMEDIATE_SR(SCTLR_EL1);
> > +DECLARE_IMMEDIATE_SR(ACTLR_EL1);
> > +DECLARE_IMMEDIATE_SR(CPACR_EL1);
> > +DECLARE_IMMEDIATE_SR(TTBR0_EL1);
> > +DECLARE_IMMEDIATE_SR(TTBR1_EL1);
> > +DECLARE_IMMEDIATE_SR(TCR_EL1);
> > +DECLARE_IMMEDIATE_SR(ESR_EL1);
> > +DECLARE_IMMEDIATE_SR(AFSR0_EL1);
> > +DECLARE_IMMEDIATE_SR(AFSR1_EL1);
> > +DECLARE_IMMEDIATE_SR(FAR_EL1);
> > +DECLARE_IMMEDIATE_SR(MAIR_EL1);
> > +DECLARE_IMMEDIATE_SR(VBAR_EL1);
> > +DECLARE_IMMEDIATE_SR(CONTEXTIDR_EL1);
> > +DECLARE_IMMEDIATE_SR(TPIDR_EL0);
> > +DECLARE_IMMEDIATE_SR(TPIDRRO_EL0);
> > +DECLARE_IMMEDIATE_SR(TPIDR_EL1);
> > +DECLARE_IMMEDIATE_SR(AMAIR_EL1);
> > +DECLARE_IMMEDIATE_SR(CNTKCTL_EL1);
> > +DECLARE_IMMEDIATE_SR(PAR_EL1);
> > +DECLARE_IMMEDIATE_SR(MDSCR_EL1);
> > +DECLARE_IMMEDIATE_SR(MDCCINT_EL1);
> > +DECLARE_IMMEDIATE_SR(PMCR_EL0);
> > +DECLARE_IMMEDIATE_SR(PMSELR_EL0);
> > +DECLARE_IMMEDIATE_SR(PMEVCNTR0_EL0);
> > +/* PMEVCNTR30_EL0 */
> > +DECLARE_IMMEDIATE_SR(PMCCNTR_EL0);
> > +DECLARE_IMMEDIATE_SR(PMEVTYPER0_EL0);
> > +/* PMEVTYPER30_EL0 */
> > +DECLARE_IMMEDIATE_SR(PMCCFILTR_EL0);
> > +DECLARE_IMMEDIATE_SR(PMCNTENSET_EL0);
> > +DECLARE_IMMEDIATE_SR(PMINTENSET_EL1);
> > +DECLARE_IMMEDIATE_SR(PMOVSSET_EL0);
> > +DECLARE_IMMEDIATE_SR(PMSWINC_EL0);
> > +DECLARE_IMMEDIATE_SR(PMUSERENR_EL0);
> > +DECLARE_IMMEDIATE_SR(DACR32_EL2);
> > +DECLARE_IMMEDIATE_SR(IFSR32_EL2);
> > +DECLARE_IMMEDIATE_SR(FPEXC32_EL2);
> > +DECLARE_IMMEDIATE_SR(DBGVCR32_EL2);
> > +
> > +static const struct sys_reg_accessor sys_reg_accessors[NR_SYS_REGS] = {
> > +	[0 ... NR_SYS_REGS - 1] = {
> > +		.rdsr = __default_read_sys_reg,
> > +		.wrsr = __default_write_sys_reg,
> > +	},
> > +
> > +	SR_HANDLER(MPIDR_EL1),
> > +	SR_HANDLER(CSSELR_EL1),
> > +	SR_HANDLER(SCTLR_EL1),
> > +	SR_HANDLER(ACTLR_EL1),
> > +	SR_HANDLER(CPACR_EL1),
> > +	SR_HANDLER(TTBR0_EL1),
> > +	SR_HANDLER(TTBR1_EL1),
> > +	SR_HANDLER(TCR_EL1),
> > +	SR_HANDLER(ESR_EL1),
> > +	SR_HANDLER(AFSR0_EL1),
> > +	SR_HANDLER(AFSR1_EL1),
> > +	SR_HANDLER(FAR_EL1),
> > +	SR_HANDLER(MAIR_EL1),
> > +	SR_HANDLER(VBAR_EL1),
> > +	SR_HANDLER(CONTEXTIDR_EL1),
> > +	SR_HANDLER(TPIDR_EL0),
> > +	SR_HANDLER(TPIDRRO_EL0),
> > +	SR_HANDLER(TPIDR_EL1),
> > +	SR_HANDLER(AMAIR_EL1),
> > +	SR_HANDLER(CNTKCTL_EL1),
> > +	SR_HANDLER(PAR_EL1),
> > +	SR_HANDLER(MDSCR_EL1),
> > +	SR_HANDLER(MDCCINT_EL1),
> > +	SR_HANDLER(PMCR_EL0),
> > +	SR_HANDLER(PMSELR_EL0),
> > +	SR_HANDLER_RANGE(PMEVCNTR0_EL0, PMEVCNTR30_EL0),
> > +	SR_HANDLER(PMCCNTR_EL0),
> > +	SR_HANDLER_RANGE(PMEVTYPER0_EL0, PMEVTYPER30_EL0),
> > +	SR_HANDLER(PMCCFILTR_EL0),
> > +	SR_HANDLER(PMCNTENSET_EL0),
> > +	SR_HANDLER(PMINTENSET_EL1),
> > +	SR_HANDLER(PMOVSSET_EL0),
> > +	SR_HANDLER(PMSWINC_EL0),
> > +	SR_HANDLER(PMUSERENR_EL0),
> > +	SR_HANDLER(DACR32_EL2),
> > +	SR_HANDLER(IFSR32_EL2),
> > +	SR_HANDLER(FPEXC32_EL2),
> > +	SR_HANDLER(DBGVCR32_EL2),
> > +};
> > +
> > +u64 vcpu_read_sys_reg(struct kvm_vcpu *vcpu, int reg)
> > +{
> > +	return sys_reg_accessors[reg].rdsr(vcpu, reg);
> > +}
> > +
> > +void vcpu_write_sys_reg(struct kvm_vcpu *vcpu, int reg, u64 val)
> > +{
> > +	sys_reg_accessors[reg].wrsr(vcpu, reg, val);
> > +}
> > +
> >  /* 3 bits per cache level, as per CLIDR, but non-existent caches always 0 */
> >  static u32 cache_levels;
> >  
> > -- 
> > 2.14.2
> > 
> > _______________________________________________
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> > kvmarm at lists.cs.columbia.edu
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