linux-kernel - Re: [PATCH] x86/sev: Add support for allowing zero SEV ASIDs.

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Message-ID: <ZZXXiFEEr7m2JitG@google.com>
Date: Wed, 3 Jan 2024 13:54:16 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Ashish Kalra <ashish.kalra@....com>
Cc: pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	thomas.lendacky@....com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	joro@...tes.org
Subject: Re: [PATCH] x86/sev: Add support for allowing zero SEV ASIDs.

On Wed, Jan 03, 2024, Ashish Kalra wrote:
> On 1/3/2024 3:10 PM, Sean Christopherson wrote:
> > > > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> > > > index d0c580607f00..bfac6d17462a 100644
> > > > --- a/arch/x86/kvm/svm/sev.c
> > > > +++ b/arch/x86/kvm/svm/sev.c
> > > > @@ -143,8 +143,20 @@ static void sev_misc_cg_uncharge(struct kvm_sev_info *sev)
> > > >    static int sev_asid_new(struct kvm_sev_info *sev)
> > > >    {
> > > > -       int asid, min_asid, max_asid, ret;
> > > > +       /*
> > > > +        * SEV-enabled guests must use asid from min_sev_asid to max_sev_asid.
> > > > +        * SEV-ES-enabled guest can use from 1 to min_sev_asid - 1.  Note, the
> > > > +        * min ASID can end up larger than the max if basic SEV support is
> > > > +        * effectively disabled by disallowing use of ASIDs for SEV guests.
> > > > +        */
> > > > +       unsigned int min_asid = sev->es_active ? 1 : min_sev_asid;
> > > > +       unsigned int max_asid = sev->es_active ? min_sev_asid - 1 : max_sev_asid;
> > > > +       unsigned int asid;
> > > >           bool retry = true;
> > > > +       int ret;
> > > > +
> > > > +       if (min_asid > max_asid)
> > > > +               return -ENOTTY;
> > > This will still return -EBUSY to user.
> > Huh?  The above is obviously -ENOTTY, and I don't see anything in the call stack
> > that will convert it to -EBUSY.
> 
> Actually, sev_asid_new() returning failure to sev_guest_init() will cause it
> to return -EBUSY to user.

Argh, I see it now.  That too should be fixed, e.g.

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index d0c580607f00..79eb11083ad5 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -246,21 +246,20 @@ static void sev_unbind_asid(struct kvm *kvm, unsigned int handle)
 static int sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp)
 {
        struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
-       int asid, ret;
+       int ret;
 
        if (kvm->created_vcpus)
                return -EINVAL;
 
-       ret = -EBUSY;
        if (unlikely(sev->active))
-               return ret;
+               return -EINVAL;
 
        sev->active = true;
        sev->es_active = argp->id == KVM_SEV_ES_INIT;
-       asid = sev_asid_new(sev);
-       if (asid < 0)
+       ret = sev_asid_new(sev);
+       if (ret < 0)
                goto e_no_asid;
-       sev->asid = asid;
+       sev->asid = ret;
 
        ret = sev_platform_init(&argp->error);
        if (ret)