linux-kernel - Re: [PATCH v8 4/9] KVM: arm/arm64: preserve host HCR

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Date:   Fri, 5 Apr 2019 12:02:29 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Amit Daniel Kachhap <amit.kachhap@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Kristina Martsenko <kristina.martsenko@....com>,
        kvmarm@...ts.cs.columbia.edu,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 4/9] KVM: arm/arm64: preserve host HCR_EL2 value

On Tue, Apr 02, 2019 at 07:57:12AM +0530, Amit Daniel Kachhap wrote:
> From: Mark Rutland <mark.rutland@....com>
> 
> 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@....com>
> [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@....com>
> Reviewed-by: James Morse <james.morse@....com>
> Cc: Marc Zyngier <marc.zyngier@....com>
> Cc: Christoffer Dall <christoffer.dall@....com>
> Cc: kvmarm@...ts.cs.columbia.edu

[...]

> 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.

>  	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.

Alternatively, just pass in the vcpu pointer (since this pattern is
already well established all over the place).

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...)

[...]

Cheers
---Dave