[xen master] x86/shadow: replace p2m_is_valid() uses

[patchbot@xxxxxxx]

commit 91772f8420dfa2fcfe4db68480c216db5b79c512
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu Mar 13 10:24:15 2025 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Mar 13 10:24:15 2025 +0100

    x86/shadow: replace p2m_is_valid() uses
    
    The justification for dropping p2m_mmio_dm from p2m_is_valid() was wrong
    for two of the shadow mode uses.
    
    In _sh_propagate() we want to create special L1 entries for p2m_mmio_dm
    pages. Hence we need to make sure we don't bail early for that type.
    
    In _sh_page_fault() we want to handle p2m_mmio_dm by forwarding to
    (internal or external) emulation. Pull the !p2m_is_mmio() check out of
    the || expression (as otherwise it would need adding to the lhs as
    well).
    
    In both cases, p2m_is_valid() in combination with p2m_is_grant() still
    doesn't cover foreign mappings. Hence use p2m_is_any_ram() plus (as
    necessary) p2m_mmio_* instead.
    
    Fixes: be59cceb2dbb ("x86/P2M: don't include MMIO_DM in p2m_is_valid()")
    Reported-by: Luca Fancellu <Luca.Fancellu@xxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Tested-by: Luca Fancellu <luca.fancellu@xxxxxxx>
---
 xen/arch/x86/mm/shadow/multi.c | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/mm/shadow/multi.c b/xen/arch/x86/mm/shadow/multi.c
index 10ddc408ff..7be9c180ec 100644
--- a/xen/arch/x86/mm/shadow/multi.c
+++ b/xen/arch/x86/mm/shadow/multi.c
@@ -471,9 +471,7 @@ _sh_propagate(struct vcpu *v,
     /* We don't shadow PAE l3s */
     ASSERT(GUEST_PAGING_LEVELS > 3 || level != 3);
 
-    /* Check there's something for the shadows to map to */
-    if ( (!p2m_is_valid(p2mt) && !p2m_is_grant(p2mt))
-         || !gfn_valid(d, target_gfn) )
+    if ( !gfn_valid(d, target_gfn) )
     {
         *sp = shadow_l1e_empty();
         goto done;
@@ -503,6 +501,13 @@ _sh_propagate(struct vcpu *v,
         goto done;
     }
 
+    /* Check there's something for the shadows to map to */
+    if ( !p2m_is_any_ram(p2mt) && p2mt != p2m_mmio_direct )
+    {
+        *sp = shadow_l1e_empty();
+        goto done;
+    }
+
     // Must have a valid target_mfn unless this is a prefetch or an l1
     // pointing at MMIO space.  In the case of a prefetch, an invalid
     // mfn means that we can not usefully shadow anything, and so we
@@ -2365,9 +2370,17 @@ static int cf_check sh_page_fault(
     gfn = guest_walk_to_gfn(&gw);
     gmfn = get_gfn(d, gfn, &p2mt);
 
+    /*
+     * p2m_mmio_dm in particular is handled further down, and hence can't be
+     * short-circuited here. Furthermore, while not fitting with architectural
+     * behavior, propagating #PF to the guest when a sensible shadow entry
+     * can't be written is necessary. Without doing so (by installing a non-
+     * present entry) we'd get back right here immediately afterwards, thus
+     * preventing the guest from making further forward progress.
+     */
     if ( shadow_mode_refcounts(d) &&
-         ((!p2m_is_valid(p2mt) && !p2m_is_grant(p2mt)) ||
-          (!p2m_is_mmio(p2mt) && !mfn_valid(gmfn))) )
+         !p2m_is_mmio(p2mt) &&
+         (!p2m_is_any_ram(p2mt) || !mfn_valid(gmfn)) )
     {
         perfc_incr(shadow_fault_bail_bad_gfn);
         SHADOW_PRINTK("BAD gfn=%"SH_PRI_gfn" gmfn=%"PRI_mfn"\n",
--
generated by git-patchbot for /home/xen/git/xen.git#master