linux-kernel - Re: [PATCH v7 08/11] KVM: arm64: introduce vcpu->arch.debug

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Date:	Fri, 3 Jul 2015 23:46:47 +0200
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	Alex Bennée <alex.bennee@...aro.org>
Cc:	kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.cs.columbia.edu, marc.zyngier@....com,
	peter.maydell@...aro.org, agraf@...e.de, drjones@...hat.com,
	pbonzini@...hat.com, zhichao.huang@...aro.org,
	jan.kiszka@...mens.com, dahi@...ux.vnet.ibm.com,
	r65777@...escale.com, bp@...e.de, Gleb Natapov <gleb@...nel.org>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Richard Weinberger <richard@....at>,
	Andre Przywara <andre.przywara@....com>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 08/11] KVM: arm64: introduce vcpu->arch.debug_ptr

On Fri, Jul 03, 2015 at 08:14:52AM +0100, Alex Bennée wrote:
> 
> Christoffer Dall <christoffer.dall@...aro.org> writes:
> 
> > On Wed, Jul 01, 2015 at 07:29:00PM +0100, Alex Bennée wrote:
> >> This introduces a level of indirection for the debug registers. Instead
> >> of using the sys_regs[] directly we store registers in a structure in
> >> the vcpu. The new kvm_arm_reset_debug_ptr() sets the debug ptr to the
> >> guest context.
> >> 
> >> This also entails updating the sys_regs code to access this new
> >> structure. New access function have been added for each set of debug
> >> registers. The generic functions are still used for the few registers
> >> stored in the main context.
> >> 
> >> New access function pointers have been added to the sys_reg_desc
> >> structure to support the GET/SET_ONE_REG ioctl operations.
> >
> > Why is this needed?
> 
> Previously I had a hacky:
> 
> 	if (r->access == trap_debug64)
> 		return debug_get64(vcpu, r, reg, uaddr);
> 
> Which used the same offset information. Now we have a cleaner:
> 
> 	if (r->set)
> 		return (r->set)(vcpu, r, reg, uaddr);
> 
> Which accesses the structure directly, as the trap functions do:
> 
> 	__u64 *r = &vcpu->arch.vcpu_debug_state.dbg_bvr[rd->reg];
> 	if (copy_from_user(uaddr, r, KVM_REG_SIZE(reg->id)) != 0)
> 		return -EFAULT;
> 	return 0;

I get it now, had to take another look at sys_regs.c.

I think this commit message needs a littel tweaking saying "Because we
no longer give the sys_regs offset for the sys_reg_desc->reg field, but
instead the index into a debug-specific struct, the generic user-space
access code no longer works, and we are therefore forced to add specific
user_set/get functions for these registers."

At least it was hard for me to understand the rationale without this,
and I think it would be good to have for someone doing git blame later.

> 
> <snip>
> >> +#if 0
> >> +static int debug_set64(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> >> +		const struct kvm_one_reg *reg, void __user *uaddr)
> >> +{
> >> +	__u64 *r = (__u64 *) ((void * )&vcpu->arch.vcpu_debug_state + rd->reg);
> >> +	if (copy_from_user(uaddr, r, KVM_REG_SIZE(reg->id)) != 0)
> >> +		return -EFAULT;
> >> +	return 0;
> >> +}
> >> +
> >> +static int debug_get64(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> >> +	const struct kvm_one_reg *reg, void __user *uaddr)
> >> +{
> >> +	__u64 *r = (__u64 *) ((void * )&vcpu->arch.vcpu_debug_state + rd->reg);
> >> +	if (copy_to_user(uaddr, r, KVM_REG_SIZE(reg->id)) != 0)
> >> +		return -EFAULT;
> >> +	return 0;
> >> +}
> >> +#endif
> >> +
> >
> > what is this ifdef'ed block of code doing here?
> 
> Oops. Yeah looks like I missed removing that after I finished the
> re-factor. These where the old get/set functions I used.
> 
> >
> >>  int kvm_arm_sys_reg_get_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
> >>  {
> >>  	const struct sys_reg_desc *r;
> >> @@ -1303,6 +1530,9 @@ int kvm_arm_sys_reg_get_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg
> >>  	if (!r)
> >>  		return get_invariant_sys_reg(reg->id, uaddr);
> >>  
> >> +	if (r->get)
> >> +		return (r->get)(vcpu, r, reg, uaddr);
> >> +
> >>  	return reg_to_user(uaddr, &vcpu_sys_reg(vcpu, r->reg), reg->id);
> >>  }
> >>  
> >> @@ -1321,6 +1551,9 @@ int kvm_arm_sys_reg_set_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg
> >>  	if (!r)
> >>  		return set_invariant_sys_reg(reg->id, uaddr);
> >>  
> >> +	if (r->set)
> >> +		return (r->set)(vcpu, r, reg, uaddr);
> >> +
> >>  	return reg_from_user(&vcpu_sys_reg(vcpu, r->reg), uaddr, reg->id);
> >>  }
> >>  
> >> diff --git a/arch/arm64/kvm/sys_regs.h b/arch/arm64/kvm/sys_regs.h
> >> index d411e25..9265e7d 100644
> >> --- a/arch/arm64/kvm/sys_regs.h
> >> +++ b/arch/arm64/kvm/sys_regs.h
> >> @@ -55,6 +55,12 @@ struct sys_reg_desc {
> >>  
> >>  	/* Value (usually reset value) */
> >>  	u64 val;
> >> +
> >> +	/* Get/Set functions, fallback if NULL */
> >
> > Is this only meant for usersapce access or when should one use these?
> 
> Yes for GET/SET
> 
ok, see other mail for potential rename to get_user / set_user if you
like the idea.

Thanks,
-Christoffer
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