linux-kernel - Re: [PATCHv2 09/12] arm64/kvm: preserve host HCR

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Message-ID: <20180409190324.GG10904@cbox>
Date:   Mon, 9 Apr 2018 21:03:24 +0200
From:   Christoffer Dall <cdall@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, arnd@...db.de,
        catalin.marinas@....com, kvmarm@...ts.cs.columbia.edu,
        linux-arch@...r.kernel.org, marc.zyngier@....com,
        suzuki.poulose@....com, will.deacon@....com, yao.qi@....com,
        kernel-hardening@...ts.openwall.com, linux-kernel@...r.kernel.org,
        awallis@...eaurora.org
Subject: Re: [PATCHv2 09/12] arm64/kvm: preserve host HCR_EL2 value

On Mon, Apr 09, 2018 at 03:57:09PM +0100, Mark Rutland wrote:
> On Tue, Feb 06, 2018 at 01:39:15PM +0100, Christoffer Dall wrote:
> > On Mon, Nov 27, 2017 at 04:38:03PM +0000, Mark Rutland wrote:
> > > diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> > > index 525c01f48867..2205f0be3ced 100644
> > > --- a/arch/arm64/kvm/hyp/switch.c
> > > +++ b/arch/arm64/kvm/hyp/switch.c
> > > @@ -71,6 +71,8 @@ static void __hyp_text __activate_traps(struct kvm_vcpu *vcpu)
> > >  {
> > >  	u64 val;
> > >  
> > > +	vcpu->arch.host_hcr_el2 = read_sysreg(hcr_el2);
> > > +
> > 
> > Looking back at this, it seems excessive to switch this at every
> > round-trip.  I think it should be possible to have this as a single
> > global (or per-CPU) variable that gets restored directly when returning
> > from the VM.
> 
> I suspect this needs to be per-cpu, to account for heterogeneous
> systems.
> 
> I guess if we move hcr_el2 into kvm_cpu_context, that gives us a
> per-vcpu copy for guests, and a per-cpu copy for the host (in the global
> kvm_host_cpu_state).
> 
> I'll have a look at how gnarly that turns out. I'm not sure how we can
> initialise that sanely for the !VHE case to match whatever el2_setup
> did.

There's no harm in jumping down to EL2 to read a register during the
initialization phase.  All it requires is an annotation of the callee
function, and a kvm_call_hyp(), and it's actually quite fast unless you
start saving/restoring a bunch of additional system registers.  See how
we call __kvm_set_tpidr_el2() for example.

Thanks,
-Christoffer