On Thu, Jul 25, 2019 at 11:12:45AM +0800, Yang Weijiang wrote:
> From: Sean Christopherson <sean.j.christopher...@intel.com>
>
> A handful of CET MSRs are not context switched through "traditional"
> methods, e.g. VMCS or manual switching, but rather are passed through
> to the guest and are saved and restored by XSAVES/XRSTORS, i.e. the
> guest's FPU state.
>
> Load the guest's FPU state if userspace is accessing MSRs whose values
> are managed by XSAVES so that the MSR helper, e.g. vmx_{get,set}_msr(),
> can simply do {RD,WR}MSR to access the guest's value.
>
> Note that guest_cpuid_has() is not queried as host userspace is allowed
> to access MSRs that have not been exposed to the guest, e.g. it might do
> KVM_SET_MSRS prior to KVM_SET_CPUID2.
>
> Signed-off-by: Sean Christopherson <sean.j.christopher...@intel.com>
> Co-developed-by: Yang Weijiang <weijiang.y...@intel.com>
> Signed-off-by: Yang Weijiang <weijiang.y...@intel.com>
> ---
> arch/x86/kvm/x86.c | 29 ++++++++++++++++++++++++++++-
> 1 file changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fafd81d2c9ea..c657e6a56527 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -102,6 +102,8 @@ static void enter_smm(struct kvm_vcpu *vcpu);
> static void __kvm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags);
> static void store_regs(struct kvm_vcpu *vcpu);
> static int sync_regs(struct kvm_vcpu *vcpu);
> +static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu);
> +static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu);
>
> struct kvm_x86_ops *kvm_x86_ops __read_mostly;
> EXPORT_SYMBOL_GPL(kvm_x86_ops);
> @@ -2959,6 +2961,12 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct
> msr_data *msr_info)
> }
> EXPORT_SYMBOL_GPL(kvm_get_msr_common);
>
> +static bool is_xsaves_msr(u32 index)
> +{
> + return index == MSR_IA32_U_CET ||
> + (index >= MSR_IA32_PL0_SSP && index <= MSR_IA32_PL3_SSP);
> +}
> +
> /*
> * Read or write a bunch of msrs. All parameters are kernel addresses.
> *
> @@ -2969,11 +2977,30 @@ static int __msr_io(struct kvm_vcpu *vcpu, struct
> kvm_msrs *msrs,
> int (*do_msr)(struct kvm_vcpu *vcpu,
> unsigned index, u64 *data))
> {
> + bool fpu_loaded = false;
> int i;
> + u64 cet_bits = XFEATURE_MASK_CET_USER | XFEATURE_MASK_CET_KERNEL;
Dunno if the compiler will actually generate different code, but this can be a
const.
> + u64 host_xss = 0;
> +
> + for (i = 0; i < msrs->nmsrs; ++i) {
> + if (!fpu_loaded && is_xsaves_msr(entries[i].index)) {
> + if (!kvm_x86_ops->xsaves_supported() ||
> + !kvm_x86_ops->supported_xss())
The "!kvm_x86_ops->supported_xss()" is redundant with the host_xss check
below.
> + continue;
Hmm, vmx_set_msr() should be checking host_xss, arguably we should call
do_msr() and let it handle the bad MSR access. I don't have a strong
opinion either way, practically speaking the end result will be the same.
If we do want to handle a misbehaving userspace here, this should be
'break' instead of 'continue'.
> +
> + host_xss = kvm_x86_ops->supported_xss();
>
> - for (i = 0; i < msrs->nmsrs; ++i)
> + if ((host_xss & cet_bits) != cet_bits)
I'm pretty sure this should check for either CET bit being set, not both,
e.g. I assume it's possible to enable and expose XFEATURE_MASK_CET_USER
but not XFEATURE_MASK_CET_KERNEL.
So something like
const u64 cet_bits = XFEATURE_MASK_CET_USER | XFEATURE_MASK_CET_KERNEL;
const bool cet_supported = kvm_x86_ops->xsaves_supported() &&
(kvm_x86_ops->supported_xss() & cet_bits);
for (i = 0; i < msrs->nmsrs; ++i) {
if (!fpu_loaded && cet_supported &&
is_xsaves_msr(entries[i].index)) {
kvm_load_guest_fpu(vcpu);
fpu_loaded = true;
}
if (do_msr(vcpu, entries[i].index, &entries[i].data))
break;
}
or
const u64 cet_bits = XFEATURE_MASK_CET_USER | XFEATURE_MASK_CET_KERNEL;
for (i = 0; i < msrs->nmsrs; ++i) {
if (!fpu_loaded && is_xsaves_msr(entries[i].index)) {
if (!kvm_x86_ops->supported_xss() ||
!(kvm_x86_ops->supported_xss() & cet_bits))
break;
kvm_load_guest_fpu(vcpu);
fpu_loaded = true;
}
if (do_msr(vcpu, entries[i].index, &entries[i].data))
break;
}
> + continue;
> +
> + kvm_load_guest_fpu(vcpu);
> + fpu_loaded = true;
> + }
> if (do_msr(vcpu, entries[i].index, &entries[i].data))
> break;
> + }
> + if (fpu_loaded)
> + kvm_put_guest_fpu(vcpu);
>
> return i;
> }
> --
> 2.17.2
>
