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    SubjectRe: [PATCH v3 09/19] x86/cpu: Clear VMX feature flag if VMX is not fully enabled
    On Thu, Nov 21, 2019 at 05:24:52PM +0100, Borislav Petkov wrote:
    > On Mon, Nov 18, 2019 at 07:12:30PM -0800, Sean Christopherson wrote:
    > > Now that the IA32_FEATURE_CONTROL MSR is guaranteed to be configured and
    > > locked, clear the VMX capability flag if the IA32_FEATURE_CONTROL MSR is
    > > not supported or if BIOS disabled VMX, i.e. locked IA32_FEATURE_CONTROL
    > > and did not set the appropriate VMX enable bit.
    > >
    > > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
    > > ---
    > > arch/x86/kernel/cpu/feature_control.c | 28 ++++++++++++++++++++++++---
    > > 1 file changed, 25 insertions(+), 3 deletions(-)
    > >
    > > diff --git a/arch/x86/kernel/cpu/feature_control.c b/arch/x86/kernel/cpu/feature_control.c
    > > index 33c9444dda52..2bd1a9e6021a 100644
    > > --- a/arch/x86/kernel/cpu/feature_control.c
    > > +++ b/arch/x86/kernel/cpu/feature_control.c
    > > @@ -5,15 +5,26 @@
    > > #include <asm/msr-index.h>
    > > #include <asm/processor.h>
    > >
    > > +#undef pr_fmt
    > > +#define pr_fmt(fmt) "x86/cpu: " fmt
    > > +
    > > +#define FEAT_CTL_UNSUPPORTED_MSG "IA32_FEATURE_CONTROL MSR unsupported on VMX capable CPU, suspected hardware or hypervisor issue.\n"
    > > +
    > > void init_feature_control_msr(struct cpuinfo_x86 *c)
    > > {
    > > + bool tboot = tboot_enabled();
    > > u64 msr;
    > >
    > > - if (rdmsrl_safe(MSR_IA32_FEATURE_CONTROL, &msr))
    > > + if (rdmsrl_safe(MSR_IA32_FEATURE_CONTROL, &msr)) {
    > > + if (cpu_has(c, X86_FEATURE_VMX)) {
    > > + pr_err_once(FEAT_CTL_UNSUPPORTED_MSG);
    > > + clear_cpu_cap(c, X86_FEATURE_VMX);
    > > + }
    > > return;
    > > + }
    >
    > Right, so this test: is this something that could happen on some
    > configurations - i.e., the MSR is not there but VMX bit is set - or are
    > you being too cautious here?

    Probably being overly cautious.

    > IOW, do you have any concrete use cases in mind (cloud provider can f*ck
    > it up this way) or?

    Yes, VMM somehow managing to break things. Admittedly extremely unlikely
    given how long IA32_FEATURE_CONTROL has been around.

    > My angle is that if this is never going to happen, why even bother to
    > print anything...

    My thought was to add an equivalent of the WARN that fires when an MSR
    access unexpectedly faults. That's effectively what'd be happening, except
    I used the safe variant to reduce the maintenance cost, e.g. so that the
    RDMSR doesn't have to be conditioned on every possible feature.

    What about a WARN_ON cpu_has? That'd be more aligned with the unexpected
    #GP on RDMSR behavior.

    if (rdmsrl_safe(...)) {
    if (WARN_ON_ONCE(cpu_has(c, X86_FEATURE_VMX)))
    clear_cpu_cap(c, X86_FEATURE_VMX);
    return;
    }

    I'm also ok dropping it altogether, though from a KVM developer
    perspective I wouldn't mind the extra sanity check :-)

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