Re: [PATCH 2/2] x86/hyperv: Move required MSRs check to initial platform probing

[Wei Liu]

On Fri, Oct 29, 2021 at 11:20:49AM +0200, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@xxxxxxxxxx> writes:
> 
> > Explicitly check for MSR_HYPERCALL and MSR_VP_INDEX support when probing
> > for running as a Hyper-V guest instead of waiting until hyperv_init() to
> > detect the bogus configuration.  Add messages to give the admin a heads
> > up that they are likely running on a broken virtual machine setup.
> >
> > At best, silently disabling Hyper-V is confusing and difficult to debug,
> > e.g. the kernel _says_ it's using all these fancy Hyper-V features, but
> > always falls back to the native versions.  At worst, the half baked setup
> > will crash/hang the kernel.
> >
> > Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---
> >  arch/x86/hyperv/hv_init.c      |  9 +--------
> >  arch/x86/kernel/cpu/mshyperv.c | 20 +++++++++++++++-----
> >  2 files changed, 16 insertions(+), 13 deletions(-)
> >
> > diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> > index 6cc845c026d4..abfb09610e22 100644
> > --- a/arch/x86/hyperv/hv_init.c
> > +++ b/arch/x86/hyperv/hv_init.c
> > @@ -347,20 +347,13 @@ static void __init hv_get_partition_id(void)
> >   */
> >  void __init hyperv_init(void)
> >  {
> > -	u64 guest_id, required_msrs;
> > +	u64 guest_id;
> >  	union hv_x64_msr_hypercall_contents hypercall_msr;
> >  	int cpuhp;
> >  
> >  	if (x86_hyper_type != X86_HYPER_MS_HYPERV)
> >  		return;
> >  
> > -	/* Absolutely required MSRs */
> > -	required_msrs = HV_MSR_HYPERCALL_AVAILABLE |
> > -		HV_MSR_VP_INDEX_AVAILABLE;
> > -
> > -	if ((ms_hyperv.features & required_msrs) != required_msrs)
> > -		return;
> > -
> >  	if (hv_common_init())
> >  		return;
> >  
> > diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> > index e095c28d27ae..ef6316fef99f 100644
> > --- a/arch/x86/kernel/cpu/mshyperv.c
> > +++ b/arch/x86/kernel/cpu/mshyperv.c
> > @@ -163,12 +163,22 @@ static uint32_t  __init ms_hyperv_platform(void)
> >  	cpuid(HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS,
> >  	      &eax, &hyp_signature[0], &hyp_signature[1], &hyp_signature[2]);
> >  
> > -	if (eax >= HYPERV_CPUID_MIN &&
> > -	    eax <= HYPERV_CPUID_MAX &&
> > -	    !memcmp("Microsoft Hv", hyp_signature, 12))
> > -		return HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS;
> > +	if (eax < HYPERV_CPUID_MIN || eax > HYPERV_CPUID_MAX ||
> > +	    memcmp("Microsoft Hv", hyp_signature, 12))
> > +		return 0;
> >  
> > -	return 0;
> > +	/* HYPERCALL and VP_INDEX MSRs are mandatory for all features. */
> > +	eax = cpuid_eax(HYPERV_CPUID_FEATURES);
> > +	if (!(eax & HV_MSR_HYPERCALL_AVAILABLE)) {
> > +		pr_warn("x86/hyperv: HYPERCALL MSR not available.\n");
> > +		return 0;
> > +	}
> > +	if (!(eax & HV_MSR_VP_INDEX_AVAILABLE)) {
> > +		pr_warn("x86/hyperv: VP_INDEX MSR not available.\n");
> > +		return 0;
> > +	}
> > +
> > +	return HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS;
> >  }
> >  
> >  static unsigned char hv_get_nmi_reason(void)
> 
> In theory, we can get away without VP_INDEX MSR as e.g. PV spinlocks
> don't need it but it will require us to add the check to all other
> features which actually need it and disable them so it's probably not
> worth the effort (and unless we're running on KVM/QEMU which actually
> *can* create such configuration, it's likely impossible to meet such
> setup in real life).
> 

Indeed. There is no need to make things more complicated than necessary.

> Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

Thanks for reviewing.

Wei.


> 
> -- 
> Vitaly
>