Re: [XEN][PATCH] xen/x86: guest_access: optimize raw_x_guest() for PV and HVM combinations

[Grygorii Strashko <grygorii_strashko@xxxxxxxx>]

On 06.11.25 19:27, Teddy Astie wrote:
> Le 06/11/2025 à 18:00, Jason Andryuk a écrit :
> > On 2025-11-06 11:33, Grygorii Strashko wrote:
> > > Hi Teddy, Jan,
> > >
> > > On 06.11.25 17:57, Teddy Astie wrote:
> > > > Le 31/10/2025 à 22:25, Grygorii Strashko a écrit :
> > > Can try.
> > Yes, I was thinking something like Teddy suggested:
> >
> > #define raw_copy_to_guest(dst, src, len)        \
> >           (is_hvm_vcpu(current) ? copy_to_user_hvm(dst, src, len) :
> >            is_pv_vcpu(current) ? copy_to_guest_pv(dst, src, len) :
> >            fail_copy(dst, src, len))
> >
> > But that made the think the is_{hvm,pv}_{vcpu,domain}() could be
> > optimized for when only 1 of HVM or PV is enabled.
> >
> > Regards,
> > Jason
> >
> > xen: Optimize is_hvm/pv_domain() for single domain type
> >
> > is_hvm_domain() and is_pv_domain() hardcode the false conditions for
> > HVM=n and PV=n, but they still leave the XEN_DOMCTL_CDF_hvm flag
> > checking.  When only one of PV or HVM is set, the result can be hard
> > coded since the other is impossible.  Notably, this removes the
> > evaluate_nospec() lfences.
> >
> > Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
> > ---
> > Untested.
> >
> > HVM=y PV=n bloat-o-meter:
> >
> > add/remove: 3/6 grow/shrink: 19/212 up/down: 3060/-60349 (-57289)
> >
> > Full bloat-o-meter below.
> > ---
> >    xen/include/xen/sched.h | 18 ++++++++++++++----
> >    1 file changed, 14 insertions(+), 4 deletions(-)
> >
> > diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
> > index f680fb4fa1..12f10d9cc8 100644
> > --- a/xen/include/xen/sched.h
> > +++ b/xen/include/xen/sched.h
> > @@ -1176,8 +1176,13 @@ static always_inline bool
> > is_hypercall_target(const struct domain *d)
> >
> >    static always_inline bool is_pv_domain(const struct domain *d)
> >    {
> > -    return IS_ENABLED(CONFIG_PV) &&
> > -        evaluate_nospec(!(d->options & XEN_DOMCTL_CDF_hvm));
> > +    if ( !IS_ENABLED(CONFIG_PV) )
> > +        return false;
> > +
> > +    if ( IS_ENABLED(CONFIG_HVM) )
> > +        return evaluate_nospec(!(d->options & XEN_DOMCTL_CDF_hvm));
> > +
> > +    return true;
> >    }
> >
> >    static always_inline bool is_pv_vcpu(const struct vcpu *v)
> > @@ -1218,8 +1223,13 @@ static always_inline bool is_pv_64bit_vcpu(const
> > struct vcpu *v)
> >
> >    static always_inline bool is_hvm_domain(const struct domain *d)
> >    {
> > -    return IS_ENABLED(CONFIG_HVM) &&
> > -        evaluate_nospec(d->options & XEN_DOMCTL_CDF_hvm);
> > +    if ( !IS_ENABLED(CONFIG_HVM) )
> > +        return false;
> > +
> > +    if ( IS_ENABLED(CONFIG_PV) )
> > +        return evaluate_nospec(d->options & XEN_DOMCTL_CDF_hvm);
> > +
> > +    return true;
> >    }
> >
> >    static always_inline bool is_hvm_vcpu(const struct vcpu *v)
> While I like the idea, it may slightly impact some logic as special
> domains (dom_xen and dom_io) are now considered HVM domains (when !PV &&
> HVM) instead of "neither PV nor HVM".
> We want at least to make sure we're not silently breaking something
> elsewhere.
first of all idle domain - I've tried to constify is_hvm_domain() and even made 
it work,
but diff is very fragile.

Diff below - just FYI.

--
Best regards,
-grygorii

Author: Grygorii Strashko <grygorii_strashko@xxxxxxxx>
Date:   Fri Oct 17 17:21:29 2025 +0300

    HACK: hvm only
    
    Signed-off-by: Grygorii Strashko <grygorii_strashko@xxxxxxxx>
diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index d65c2bd3661f..2ea3d81670de 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -567,17 +567,17 @@ int arch_vcpu_create(struct vcpu *v)
 
     spin_lock_init(&v->arch.vpmu.vpmu_lock);
 
-    if ( is_hvm_domain(d) )
-        rc = hvm_vcpu_initialise(v);
-    else if ( !is_idle_domain(d) )
-        rc = pv_vcpu_initialise(v);
-    else
+    if ( is_idle_domain(d) )
     {
         /* Idle domain */
         v->arch.cr3 = __pa(idle_pg_table);
         rc = 0;
         v->arch.msrs = ZERO_BLOCK_PTR; /* Catch stray misuses */
     }
+    else if ( is_hvm_domain(d) )
+        rc = hvm_vcpu_initialise(v);
+    else
+        rc = pv_vcpu_initialise(v);
 
     if ( rc )
         goto fail;
@@ -2123,7 +2123,7 @@ void context_switch(struct vcpu *prev, struct vcpu *next)
     vpmu_switch_from(prev);
     np2m_schedule(NP2M_SCHEDLE_OUT);
 
-    if ( is_hvm_domain(prevd) && !list_empty(&prev->arch.hvm.tm_list) )
+    if ( !is_idle_domain(prevd) && is_hvm_domain(prevd) && 
!list_empty(&prev->arch.hvm.tm_list) )
         pt_save_timer(prev);
 
     local_irq_disable();
diff --git a/xen/arch/x86/hvm/Kconfig b/xen/arch/x86/hvm/Kconfig
index 79c5bcbb3a24..533ad71d1018 100644
--- a/xen/arch/x86/hvm/Kconfig
+++ b/xen/arch/x86/hvm/Kconfig
@@ -126,4 +126,8 @@ config VHPET
 
       If unsure, say Y.
 
+config HVM_ONLY
+    bool "Only HVM/PVH"
+    default y
+
 endif
diff --git a/xen/arch/x86/hvm/svm/vmcb.c b/xen/arch/x86/hvm/svm/vmcb.c
index 839d3ff91b5a..e3c9b4ffba52 100644
--- a/xen/arch/x86/hvm/svm/vmcb.c
+++ b/xen/arch/x86/hvm/svm/vmcb.c
@@ -236,7 +236,7 @@ static void cf_check vmcb_dump(unsigned char ch)
 
     for_each_domain ( d )
     {
-        if ( !is_hvm_domain(d) )
+        if ( is_idle_domain(d) || !is_hvm_domain(d) )
             continue;
         printk("\n>>> Domain %d <<<\n", d->domain_id);
         for_each_vcpu ( d, v )
diff --git a/xen/arch/x86/mm/p2m-basic.c b/xen/arch/x86/mm/p2m-basic.c
index e126fda26760..c53269b3c06d 100644
--- a/xen/arch/x86/mm/p2m-basic.c
+++ b/xen/arch/x86/mm/p2m-basic.c
@@ -34,7 +34,7 @@ static int p2m_initialise(struct domain *d, struct p2m_domain 
*p2m)
     p2m->default_access = p2m_access_rwx;
     p2m->p2m_class = p2m_host;
 
-    if ( !is_hvm_domain(d) )
+    if ( is_idle_domain(d) || !is_hvm_domain(d) )
         return 0;
 
     p2m_pod_init(p2m);
@@ -113,7 +113,7 @@ int p2m_init(struct domain *d)
     int rc;
 
     rc = p2m_init_hostp2m(d);
-    if ( rc || !is_hvm_domain(d) )
+    if ( rc || is_idle_domain(d) || !is_hvm_domain(d) )
         return rc;
 
     /*
diff --git a/xen/arch/x86/mm/p2m-pod.c b/xen/arch/x86/mm/p2m-pod.c
index 05633fe2ac88..4e62d98861fe 100644
--- a/xen/arch/x86/mm/p2m-pod.c
+++ b/xen/arch/x86/mm/p2m-pod.c
@@ -1425,7 +1425,7 @@ bool p2m_pod_active(const struct domain *d)
     struct p2m_domain *p2m;
     bool res;
 
-    if ( !is_hvm_domain(d) )
+    if ( is_idle_domain(d) || !is_hvm_domain(d) )
         return false;
 
     p2m = p2m_get_hostp2m(d);
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index ccf8563e5a64..e1862c5085f5 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -2158,7 +2158,7 @@ static int __hwdom_init cf_check io_bitmap_cb(
 
 void __hwdom_init setup_io_bitmap(struct domain *d)
 {
-    if ( !is_hvm_domain(d) )
+    if ( is_idle_domain(d) || !is_hvm_domain(d) )
         return;
 
     bitmap_fill(d->arch.hvm.io_bitmap, 0x10000);
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index 3764b58c9ccf..b1fb67b35d0f 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -1214,8 +1214,8 @@ static always_inline bool is_pv_64bit_vcpu(const struct 
vcpu *v)
 
 static always_inline bool is_hvm_domain(const struct domain *d)
 {
-    return IS_ENABLED(CONFIG_HVM) &&
-        evaluate_nospec(d->options & XEN_DOMCTL_CDF_hvm);
+    return IS_ENABLED(CONFIG_HVM_ONLY) || (IS_ENABLED(CONFIG_HVM) &&
+        evaluate_nospec(d->options & XEN_DOMCTL_CDF_hvm));
 }
 
 static always_inline bool is_hvm_vcpu(const struct vcpu *v)