Re: [PATCH v8 4/9] KVM: arm/arm64: preserve host HCR_EL2 value

[Amit Daniel Kachhap]

Hi,

On 4/5/19 4:32 PM, Dave Martin wrote:
> On Tue, Apr 02, 2019 at 07:57:12AM +0530, Amit Daniel Kachhap wrote:
> > From: Mark Rutland <mark.rutland@xxxxxxx>
> >
> > When restoring HCR_EL2 for the host, KVM uses HCR_HOST_VHE_FLAGS, which
> > is a constant value. This works today, as the host HCR_EL2 value is
> > always the same, but this will get in the way of supporting extensions
> > that require HCR_EL2 bits to be set conditionally for the host.
> >
> > To allow such features to work without KVM having to explicitly handle
> > every possible host feature combination, this patch has KVM save/restore
> > for the host HCR when switching to/from a guest HCR. The saving of the
> > register is done once during cpu hypervisor initialization state and is
> > just restored after switch from guest.
> >
> > For fetching HCR_EL2 during kvm initialisation, a hyp call is made using
> > kvm_call_hyp and is helpful in non-VHE case.
> >
> > For the hyp TLB maintenance code, __tlb_switch_to_host_vhe() is updated
> > to toggle the TGE bit with a RMW sequence, as we already do in
> > __tlb_switch_to_guest_vhe().
> >
> > The value of hcr_el2 is now stored in struct kvm_cpu_context as both host
> > and guest can now use this field in a common way.
> >
> > Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> > [Added cpu_init_host_ctxt, hcr_el2 field in struct kvm_cpu_context,
> > save hcr_el2 in hyp init stage]
> > Signed-off-by: Amit Daniel Kachhap <amit.kachhap@xxxxxxx>
> > Reviewed-by: James Morse <james.morse@xxxxxxx>
> > Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> > Cc: Christoffer Dall <christoffer.dall@xxxxxxx>
> > Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx
>
> [...]
>
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index a01fe087..3b09fd0 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -209,6 +209,8 @@ struct kvm_cpu_context {
> >   		u32 copro[NR_COPRO_REGS];
> >   	};
> >   
> > +	/* HYP host/guest configuration */
> > +	u64 hcr_el2;
>
> Minor nit: You could delete "host/guest" from the comment here.  This is
> implied by the fact that the member is in struct kvm_cpu_context in the
> first place.
ok. Agree with you.
> >   	struct kvm_vcpu *__hyp_running_vcpu;
> >   };
>
> [...]
>
> > diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> > index 3563fe6..f5cefa1 100644
> > --- a/arch/arm64/kvm/hyp/switch.c
> > +++ b/arch/arm64/kvm/hyp/switch.c
>
> [...]
>
> > @@ -159,9 +159,10 @@ static void deactivate_traps_vhe(void)
> >   }
> >   NOKPROBE_SYMBOL(deactivate_traps_vhe);
> >   
> > -static void __hyp_text __deactivate_traps_nvhe(void)
> > +static void __hyp_text __deactivate_traps_nvhe(struct kvm_cpu_context *host_ctxt)
>
> Where __hyp_text functions accept pointer arguments, they are usually
> hyp pointers already...  (see below)
>
> >   {
> >   	u64 mdcr_el2 = read_sysreg(mdcr_el2);
> > +	struct kvm_cpu_context *hyp_host_ctxt = kern_hyp_va(host_ctxt);
> >   
> >   	__deactivate_traps_common();
> >   
> > @@ -169,25 +170,28 @@ static void __hyp_text __deactivate_traps_nvhe(void)
> >   	mdcr_el2 |= MDCR_EL2_E2PB_MASK << MDCR_EL2_E2PB_SHIFT;
> >   
> >   	write_sysreg(mdcr_el2, mdcr_el2);
> > -	write_sysreg(HCR_HOST_NVHE_FLAGS, hcr_el2);
> > +	write_sysreg(hyp_host_ctxt->hcr_el2, hcr_el2);
> >   	write_sysreg(CPTR_EL2_DEFAULT, cptr_el2);
> >   }
> >   
> >   static void __hyp_text __deactivate_traps(struct kvm_vcpu *vcpu)
> >   {
> > +	struct kvm_cpu_context *host_ctxt;
> > +
> > +	host_ctxt = vcpu->arch.host_cpu_context;
>
> host_ctxt is not otherwise used here, so can we convert it up-front so
> that the argument to __deactivate_traps_nvhe() and
> deactivate_traps_vhe() is a hyp pointer already?
>
> So:
>
> 	struct kvm_cpu_context *hyp_host_ctxt;
>
> 	hyp_host_ctxt = kern_hyp_va(vcpu->arch.host_cpu_context);
>
> >   	/*
> >   	 * If we pended a virtual abort, preserve it until it gets
> >   	 * cleared. See D1.14.3 (Virtual Interrupts) for details, but
> >   	 * the crucial bit is "On taking a vSError interrupt,
> >   	 * HCR_EL2.VSE is cleared to 0."
> >   	 */
> > -	if (vcpu->arch.hcr_el2 & HCR_VSE)
> > -		vcpu->arch.hcr_el2 = read_sysreg(hcr_el2);
> > +	if (vcpu->arch.ctxt.hcr_el2 & HCR_VSE)
> > +		vcpu->arch.ctxt.hcr_el2 = read_sysreg(hcr_el2);
> >   
> >   	if (has_vhe())
> > -		deactivate_traps_vhe();
> > +		deactivate_traps_vhe(host_ctxt);
> >   	else
> > -		__deactivate_traps_nvhe();
> > +		__deactivate_traps_nvhe(host_ctxt);
>
> Then just pass hyp_host_ctxt to both of these, and drop the
> kern_hyp_va() conversion from __deactivate_traps_nvhe().
>
> This may be a bit less confusing.
Yes your explanation makes sense.
> Alternatively, just pass in the vcpu pointer (since this pattern is
> already well established all over the place).
I think passing vcpu as parameter will make it consistent with other 
existing functions. __kvm_vcpu_run_nvhe function also takes vcpu and 
extracts hyp_host_ctxt.
> Another option could be to pull the hcr_el2 write out of the backends
> entirely and put it in this common code instead.  This doesn't look
> straightforward though (or at least, I don't remember enough about how
> all these traps handling functions fit together...)
ok.

Thanks,
Amit D
> [...]
>
> Cheers
> ---Dave