linux-kernel - Re: [PATCH v2 1/2] KVM: x86: Check hypercall's exit to userspace generically

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Message-ID: <20240813061451.obyytu2rrm6mrmqj@yy-desk-7060>
Date: Tue, 13 Aug 2024 14:14:51 +0800
From: Yuan Yao <yuan.yao@...ux.intel.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, pbonzini@...hat.com,
	seanjc@...gle.com, isaku.yamahata@...el.com,
	rick.p.edgecombe@...el.com, michael.roth@....com
Subject: Re: [PATCH v2 1/2] KVM: x86: Check hypercall's exit to userspace
 generically

On Tue, Aug 13, 2024 at 01:56:14PM +0800, Yuan Yao wrote:
> On Tue, Aug 13, 2024 at 01:12:55PM +0800, Binbin Wu wrote:
> > Check whether a KVM hypercall needs to exit to userspace or not based on
> > hypercall_exit_enabled field of struct kvm_arch.
> >
> > Userspace can request a hypercall to exit to userspace for handling by
> > enable KVM_CAP_EXIT_HYPERCALL and the enabled hypercall will be set in
> > hypercall_exit_enabled.  Make the check code generic based on it.
> >
> > Signed-off-by: Binbin Wu <binbin.wu@...ux.intel.com>
> > ---
> >  arch/x86/kvm/x86.c | 4 ++--
> >  arch/x86/kvm/x86.h | 7 +++++++
> >  2 files changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index af6c8cf6a37a..6e16c9751af7 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -10226,8 +10226,8 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> >  	cpl = kvm_x86_call(get_cpl)(vcpu);
> >
> >  	ret = __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl);
> > -	if (nr == KVM_HC_MAP_GPA_RANGE && !ret)
> > -		/* MAP_GPA tosses the request to the user space. */
> > +	if (!ret && is_kvm_hc_exit_enabled(vcpu->kvm, nr))
> > +		/* The hypercall is requested to exit to userspace. */
> >  		return 0;
> >
> >  	if (!op_64_bit)
> > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> > index 50596f6f8320..0cbec76b42e6 100644
> > --- a/arch/x86/kvm/x86.h
> > +++ b/arch/x86/kvm/x86.h
> > @@ -547,4 +547,11 @@ int kvm_sev_es_string_io(struct kvm_vcpu *vcpu, unsigned int size,
> >  			 unsigned int port, void *data,  unsigned int count,
> >  			 int in);
> >
> > +static inline bool is_kvm_hc_exit_enabled(struct kvm *kvm, unsigned long hc_nr)
> > +{
> > +	if(WARN_ON_ONCE(hc_nr >= sizeof(kvm->arch.hypercall_exit_enabled) * 8))
>
> How about:
>
> if (!(BIT(hc_nr) & KVM_EXIT_HYPERCALL_VALID_MASK))
>
> KVM_EXIT_HYPERCALL_VALID_MASK is used to guard kvm->arch.hypercall_exit_enabled
> on KVM_CAP_EXIT_HYPERCALL, "hc_nr > maximum supported hc" AND "hc_nr <=
> bit_count(kvm->arch.hypercall_exit_enabled)" should be treated as invalid yet to
> me.

Not real good idea. Rely on hypercall_exit_enabled is good enough, this brings
unnecessary complexity.

>
> > +		return false;
> > +
> > +	return kvm->arch.hypercall_exit_enabled & (1 << hc_nr);
>
> BIT(xx) instead of "1 << hc_nr" for better readability.
>
> > +}
> >  #endif
> > --
> > 2.43.2
> >
> >
>