Re: [PATCH v6 1/6] arm64/kvm: preserve host HCR_EL2 value

From: Mark Rutland <mark.rutland@arm.com>
To: Amit Daniel Kachhap <amit.kachhap@arm.com>
Cc: Andrew Jones <drjones@redhat.com>,
	linux-kernel@vger.kernel.org,
	Julien Thierry <julien.thierry@arm.com>,
	Marc Zyngier <marc.zyngier@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Christoffer Dall <christoffer.dall@arm.com>,
	Kristina Martsenko <kristina.martsenko@arm.com>,
	kvmarm@lists.cs.columbia.edu, James Morse <james.morse@arm.com>,
	Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>,
	Dave Martin <Dave.Martin@arm.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v6 1/6] arm64/kvm: preserve host HCR_EL2 value
Date: Thu, 21 Feb 2019 11:50:34 +0000	[thread overview]
Message-ID: <20190221115034.GA33673@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <1550568271-5319-2-git-send-email-amit.kachhap@arm.com>

Hi,

On Tue, Feb 19, 2019 at 02:54:26PM +0530, Amit Daniel Kachhap wrote:
> From: Mark Rutland <mark.rutland@arm.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 NHVE 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@arm.com>
> [Added __cpu_copy_hyp_conf, hcr_el2 field in struct kvm_cpu_context]
> Signed-off-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> Cc: Christoffer Dall <christoffer.dall@arm.com>
> Cc: kvmarm@lists.cs.columbia.edu

[...]

> +/**
> + * __cpu_copy_hyp_conf - copy the boot hyp configuration registers
> + *
> + * It is called once per-cpu during CPU hyp initialisation.
> + */
> +static inline void __cpu_copy_hyp_conf(void)

I think this would be better named as something like:

  cpu_init_host_ctxt()

... as that makes it a bit clearer as to what is being initialized.

[...]

> +/**
> + * __kvm_populate_host_regs - Stores host register values
> + *
> + * This function acts as a function handler parameter for kvm_call_hyp and
> + * may be called from EL1 exception level to fetch the register value.
> + */
> +void __hyp_text __kvm_populate_host_regs(void)
> +{
> +	struct kvm_cpu_context *host_ctxt;
> +
> +	if (has_vhe())
> +		host_ctxt = this_cpu_ptr(&kvm_host_cpu_state);
> +	else
> +		host_ctxt = __hyp_this_cpu_ptr(kvm_host_cpu_state);

Do we need the has_vhe() check here?

Can't we always do:

	host_ctxt = __hyp_this_cpu_ptr(kvm_host_cpu_state);

... regardless of VHE? Or is that broken for VHE somehow?

Thanks,
Mark.

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