On Wed, 29 Sep 2021 11:35:46 +0100,
Suzuki K Poulose <suzuki.poulose@xxxxxxx> wrote:
>
> On 03/09/2021 10:16, Marc Zyngier wrote:
> > Although KVM can be compiled out of the kernel, it cannot be disabled
> > at runtime. Allow this possibility by introducing a new mode that
> > will prevent KVM from initialising.
> >
> > This is useful in the (limited) circumstances where you don't want
> > KVM to be available (what is wrong with you?), or when you want
> > to install another hypervisor instead (good luck with that).
> >
> > Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> > ---
> > Documentation/admin-guide/kernel-parameters.txt | 3 +++
> > arch/arm64/include/asm/kvm_host.h | 1 +
> > arch/arm64/kernel/idreg-override.c | 1 +
> > arch/arm64/kvm/arm.c | 14 +++++++++++++-
> > 4 files changed, 18 insertions(+), 1 deletion(-)
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 91ba391f9b32..cc5f68846434 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -2365,6 +2365,9 @@
> > kvm-arm.mode=
> > [KVM,ARM] Select one of KVM/arm64's modes of operation.
> > + none: Forcefully disable KVM and run in nVHE
> > mode,
> > + preventing KVM from ever initialising.
> > +
> > nvhe: Standard nVHE-based mode, without support for
> > protected guests.
> > diff --git a/arch/arm64/include/asm/kvm_host.h
> > b/arch/arm64/include/asm/kvm_host.h
> > index f8be56d5342b..019490c67976 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -58,6 +58,7 @@
> > enum kvm_mode {
> > KVM_MODE_DEFAULT,
> > KVM_MODE_PROTECTED,
> > + KVM_MODE_NONE,
> > };
> > enum kvm_mode kvm_get_mode(void);
> > diff --git a/arch/arm64/kernel/idreg-override.c
> > b/arch/arm64/kernel/idreg-override.c
> > index d8e606fe3c21..57013c1b6552 100644
> > --- a/arch/arm64/kernel/idreg-override.c
> > +++ b/arch/arm64/kernel/idreg-override.c
> > @@ -95,6 +95,7 @@ static const struct {
> > char alias[FTR_ALIAS_NAME_LEN];
> > char feature[FTR_ALIAS_OPTION_LEN];
> > } aliases[] __initconst = {
> > + { "kvm-arm.mode=none", "id_aa64mmfr1.vh=0" },
> > { "kvm-arm.mode=nvhe", "id_aa64mmfr1.vh=0" },
> > { "kvm-arm.mode=protected", "id_aa64mmfr1.vh=0" },
> > { "arm64.nobti", "id_aa64pfr1.bt=0" },
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index fe102cd2e518..cdc70e238316 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -2064,6 +2064,11 @@ int kvm_arch_init(void *opaque)
> > return -ENODEV;
> > }
> > + if (kvm_get_mode() == KVM_MODE_NONE) {
> > + kvm_info("KVM disabled from command line\n");
> > + return -ENODEV;
> > + }
> > +
> > in_hyp_mode = is_kernel_in_hyp_mode();
> > if (cpus_have_final_cap(ARM64_WORKAROUND_DEVICE_LOAD_ACQUIRE)
> > ||
> > @@ -2137,8 +2142,15 @@ static int __init early_kvm_mode_cfg(char *arg)
> > return 0;
> > }
> > - if (strcmp(arg, "nvhe") == 0 &&
> > !WARN_ON(is_kernel_in_hyp_mode()))
> > + if (strcmp(arg, "nvhe") == 0 && !WARN_ON(is_kernel_in_hyp_mode())) {
> > + kvm_mode = KVM_MODE_DEFAULT;
> > return 0;
> > + }
> > +
> > + if (strcmp(arg, "none") == 0 && !WARN_ON(is_kernel_in_hyp_mode())) {
>
> nit: Does this really need to WARN here ? Unlike the "nvhe" case, if the
> user wants to keep the KVM out of the picture for, say debugging
> something, it is perfectly Ok to allow the kernel to be running at EL2
> without having to change the Firmware to alter the landing EL for the
> kernel ?
Well, the doc says "run in nVHE mode" and the option forces
id_aa64mmfr1.vh=0. The WARN_ON() will only fires on broken^Wfruity HW
that is VHE only. Note that this doesn't rely on any firmware change
(we drop from EL2 to EL1 and stay there).
We could add another option (none-vhe?) that stays at EL2 and still
disables KVM if there is an appetite for it.
> Otherwise,
>
> Acked-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
>
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
