Re: [PATCH v4 10/19] x86/cpu: Detect VMX features on Intel, Centaur and Zhaoxin CPUs

Borislav Petkov
2019-12-12T11:38:49Z

On Wed, Nov 27, 2019 at 05:40:07PM -0800, Sean Christopherson wrote:
> diff --git a/arch/x86/kernel/cpu/feat_ctl.c b/arch/x86/kernel/cpu/feat_ctl.c
> index a46c9e46f937..93268bde662a 100644
> --- a/arch/x86/kernel/cpu/feat_ctl.c
> +++ b/arch/x86/kernel/cpu/feat_ctl.c
> @@ -4,6 +4,72 @@
> #include <asm/cpufeature.h>
> #include <asm/msr-index.h>
> #include <asm/processor.h>
> +#include <asm/vmx.h>
> +
> +#ifdef CONFIG_X86_VMX_FEATURE_NAMES
> +enum vmx_feature_leafs {
> + MISC_FEATURES = 0,
> + PRIMARY_PROC_CTLS,
> + SECONDARY_PROC_CTLS,
> + NR_VMX_FEATURE_WORDS,
> +};
> +
> +#define F(x) BIT(VMX_FEATURE_##x & 0x1f)

Eww, this F-thing has been always bugging me, especially if it means
something a little different each time:

arch/x86/crypto/blowfish-x86_64-asm_64.S:59:#define F() \
arch/x86/kernel/cpu/feat_ctl.c:17:#define F(x) BIT(VMX_FEATURE_##x & 0x1f)
arch/x86/kvm/cpuid.c:65:#define F(x) bit(X86_FEATURE_##x)
arch/x86/kvm/emulate.c:4393:#define F(_f, _e) { .flags = (_f) | Fastop, .u.fastop = (_e) }
arch/x86/kvm/svm.c:5927:#define F(x) bit(X86_FEATURE_##x)

I guess you can call yours VMX_F() or so, just so that it's name is
something different.

> +static void init_vmx_capabilities(struct cpuinfo_x86 *c)
> +{
> + u32 supported, funcs, ept, vpid, ign;
> +
> + BUILD_BUG_ON(NVMXINTS != NR_VMX_FEATURE_WORDS);
> +
> + /*
> + * The high bits contain the allowed-1 settings, i.e. features that can
> + * be turned on. The low bits contain the allowed-0 settings, i.e.
> + * features that can be turned off. Ignore the allowed-0 settings,
> + * if a feature can be turned on then it's supported.
> + */
> + rdmsr(MSR_IA32_VMX_PROCBASED_CTLS, ign, supported);
> + c->vmx_capability[PRIMARY_PROC_CTLS] = supported;
> +
> + rdmsr_safe(MSR_IA32_VMX_PROCBASED_CTLS2, &ign, &supported);
> + c->vmx_capability[SECONDARY_PROC_CTLS] = supported;
> +
> + rdmsr(MSR_IA32_VMX_PINBASED_CTLS, ign, supported);
> + rdmsr_safe(MSR_IA32_VMX_VMFUNC, &ign, &funcs);
> +
> + /*
> + * Except for EPT+VPID, which enumerates support for both in a single
> + * MSR, low for EPT, high for VPID.
> + */
> + rdmsr_safe(MSR_IA32_VMX_EPT_VPID_CAP, &ept, &vpid);

Right, so this is a garden variety of rdmsr() and rdmsr_safe() and
the safe variant's retval needs to be checked, strictly speaking. It
probably doesn't matter here since you'll get 0s if it fails, which
means feature not supported, so all good.

But I guess you can still use rdmsr_safe() everywhere just so it doesn't
cause head scratching in the future, when one looks at that code.

> +#endif /* CONFIG_X86_VMX_FEATURE_NAMES */
>
> #undef pr_fmt
> #define pr_fmt(fmt) "x86/cpu: " fmt
> @@ -50,5 +116,9 @@ void init_ia32_feat_ctl(struct cpuinfo_x86 *c)
> pr_err_once("VMX (%s TXT) disabled by BIOS\n",
> tboot ? "inside" : "outside");
> clear_cpu_cap(c, X86_FEATURE_VMX);
> + } else {
> +#ifdef CONFIG_X86_VMX_FEATURE_NAMES
> + init_vmx_capabilities(c);
> +#endif

Can't say that I'm happy about all that ifdeffery but I guess we need
to perpetuate this since X86_FEATURE_NAMES is there for embedded. In
practice, probably no one disables it...

--
Regards/Gruss,
Boris.