linux-kernel - Re: [PATCH v6 1/6] arm64/kvm: preserve host HCR

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Message-ID: <01fc3209-1cc2-9ef8-be87-ea04bc44b89b@arm.com>
Date:   Mon, 25 Feb 2019 18:09:29 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Mark Rutland <mark.rutland@....com>,
        Amit Daniel Kachhap <amit.kachhap@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Christoffer Dall <christoffer.dall@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Andrew Jones <drjones@...hat.com>,
        Dave Martin <Dave.Martin@....com>,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        kvmarm@...ts.cs.columbia.edu,
        Kristina Martsenko <kristina.martsenko@....com>,
        linux-kernel@...r.kernel.org, James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry@....com>
Subject: Re: [PATCH v6 1/6] arm64/kvm: preserve host HCR_EL2 value

On 21/02/2019 11:50, Mark Rutland wrote:
> Hi,
> 
> On Tue, Feb 19, 2019 at 02:54:26PM +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 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@....com>
>> [Added __cpu_copy_hyp_conf, hcr_el2 field in struct kvm_cpu_context]
>> Signed-off-by: Amit Daniel Kachhap <amit.kachhap@....com>
>> Cc: Marc Zyngier <marc.zyngier@....com>
>> Cc: Christoffer Dall <christoffer.dall@....com>
>> Cc: kvmarm@...ts.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?

The whole point of __hyp_this_cpu_ptr is that it is always valid...
See 85478bab40917 for details.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...