On Wed, Apr 15, 2015 at 4:52 PM, Benjamin Serebrin <serebrin@xxxxxxxxxx> wrote:
> There's a bug in kvm/vmx.c: if the host enabled machine check (CR4.MCE==1),
> the value gets zeroed while the CPU is running in guest context.
> If a machine check event arrives while the CPU is in guest context and
> effective CR4.MCE is zero, the machine raises CATERR and crashes.
>
> We should make sure the host value of CR4.MCE is always active. There
> are read and write shadows for the guest to think it wrote its own value.
>
> For discussion: there's new complexity with CR4 shadowing
> (1e02ce4cccdcb9688386e5b8d2c9fa4660b45389). I measure CR4 reads at 24
> cycles on haswell and 36 on sandybridge, which compares well with
> L2 miss costs. Is the shadowing worth the complexity? CR4 is also
> cached (with no real consistency mechanism) in the VMCS at the time
> of guest VCPU creation. If there is ever a change in CR4 value
> over time, or if CR4 is different on different CPUs in the system, all this
> logic gets broken.
The cache line I chose for the shadow is hot during context switches,
so we shouldn't ever have a meaningful cache miss there (except maybe
in KVM).
Also, we do have a consistency mechanism -- see:
commit d974baa398f34393db76be45f7d4d04fbdbb4a0a
Author: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Date: Wed Oct 8 09:02:13 2014 -0700
x86,kvm,vmx: Preserve CR4 across VM entry
>
>
> ---
>
> The host's decision to enable machine check exceptions should remain
> in force during non-root mode. KVM was writing 0 to cr4 on VCPU reset
> and passed a slightly-modified 0 to the vmcs.guest_cr4 value.
>
> Tested: Inject machine check while a guest is spinning.
> Before the change, if guest CR4.MCE==0, then the machine check is
> escalated to Catastrophic Error (CATERR) and the machine dies.
> If guest CR4.MCE==1, then the machine check causes VMEXIT and is
> handled normally by host Linux. After the change, injecting a machine
> check causes normal Linux machine check handling.
>
>
> ---
> arch/x86/kvm/vmx.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index a214104..44c8d24 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -3456,8 +3456,16 @@ static void vmx_set_cr3(struct kvm_vcpu *vcpu,
> unsigned long cr3)
>
> static int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
> {
> - unsigned long hw_cr4 = cr4 | (to_vmx(vcpu)->rmode.vm86_active ?
> - KVM_RMODE_VM_CR4_ALWAYS_ON : KVM_PMODE_VM_CR4_ALWAYS_ON);
> + /*
> + * Pass through host's Machine Check Enable value to hw_cr4, which
> + * is in force while we are in guest mode. Do not let guests control
> + * this bit, even if host CR4.MCE == 0.
> + */
> + unsigned long hw_cr4 =
> + (read_cr4() & X86_CR4_MCE) |
cr4_read_shadow? There is no read_cr4 any more.
> + (cr4 & ~X86_CR4_MCE) |
> + (to_vmx(vcpu)->rmode.vm86_active ?
> + KVM_RMODE_VM_CR4_ALWAYS_ON : KVM_PMODE_VM_CR4_ALWAYS_ON);
>
Otherwise looks sensible.
--Andy
> if (cr4 & X86_CR4_VMXE) {
> /*
> --
--
Andy Lutomirski
AMA Capital Management, LLC
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
