linux-kernel - Re: [PATCH v2] KVM: arm64: Only set KVM_MODE_PROTECTED if is_hyp_mode

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Date:   Sat, 10 Sep 2022 14:43:44 +0100
From:   Will Deacon <will@...nel.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Elliot Berman <quic_eberman@...cinc.com>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH v2] KVM: arm64: Only set KVM_MODE_PROTECTED if
 is_hyp_mode_available()

On Sat, Sep 10, 2022 at 10:09:31AM +0100, Marc Zyngier wrote:
> On Fri, 09 Sep 2022 18:55:18 +0100,
> Elliot Berman <quic_eberman@...cinc.com> wrote:
> > 
> > 
> > 
> > On 9/9/2022 10:28 AM, Catalin Marinas wrote:
> > > On Fri, Sep 09, 2022 at 07:45:52AM -0700, Elliot Berman wrote:
> > >> Do not switch kvm_mode to KVM_MODE_PROTECTED if hypervisor mode is not
> > >> available. This prevents "Protected KVM" cpu capability being reported
> > >> when Linux is booting in EL1 and would not have KVM enabled.
> > >> 
> > >> Signed-off-by: Elliot Berman <quic_eberman@...cinc.com>
> > >> ---
> > >>   arch/arm64/kvm/arm.c | 4 +++-
> > >>   1 file changed, 3 insertions(+), 1 deletion(-)
> > >> 
> > >> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > >> index 8fe73ee5fa84..861f4b388879 100644
> > >> --- a/arch/arm64/kvm/arm.c
> > >> +++ b/arch/arm64/kvm/arm.c
> > >> @@ -2272,7 +2272,9 @@ static int __init early_kvm_mode_cfg(char *arg)
> > >>   		return -EINVAL;
> > >>     	if (strcmp(arg, "protected") == 0) {
> > >> -		if (!is_kernel_in_hyp_mode())
> > >> +		if (!is_hyp_mode_available())
> > >> +			kvm_mode = KVM_MODE_DEFAULT;
> > > 
> > > I think kvm_mode is already KVM_MODE_DEFAULT at this point. You may want
> > > to print a warning instead.
> > > 
> > 
> > Does it make sense to print warning for kvm-arm.mode=nvhe as well?
> 
> In general, specifying a kvm-arm.mode when no hypervisor mode is
> available should be reported as a warning.

As long as this is pr_warn() rather than WARN() then I agree. Otherwise,
kernels with a kvm-arm.mode hardcoded in CONFIG_CMDLINE (e.g. Android's
GKI) will make for noisy guests.

Will