[xen master] x86/msi: passthrough all MSI-X vector ctrl writes to device model

[patchbot@xxxxxxx]

commit 64b21662b1b1619b7b411365aa83ec7fca9edeb4
Author:     Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
AuthorDate: Tue Apr 30 08:31:01 2024 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Apr 30 08:31:01 2024 +0200

    x86/msi: passthrough all MSI-X vector ctrl writes to device model
    
    QEMU needs to know whether clearing maskbit of a vector is really
    clearing, or was already cleared before. Currently Xen sends only
    clearing that bit to the device model, but not setting it, so QEMU
    cannot detect it. Because of that, QEMU is working this around by
    checking via /dev/mem, but that isn't the proper approach.
    
    Give all necessary information to QEMU by passing all ctrl writes,
    including masking a vector. Advertise the new behavior via
    XENVER_get_features, so QEMU can know it doesn't need to access /dev/mem
    anymore.
    
    While this commit doesn't move the whole maskbit handling to QEMU (as
    discussed on xen-devel as one of the possibilities), it is a necessary
    first step anyway. Including telling QEMU it will get all the required
    information to do so. The actual implementation would need to include:
     - a hypercall for QEMU to control just maskbit (without (re)binding the
       interrupt again
     - a method for QEMU to tell Xen it will actually do the work
    Those are not part of this series.
    
    Signed-off-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/hvm/vmsi.c       | 19 ++++++++++++++-----
 xen/common/kernel.c           |  1 +
 xen/include/public/features.h |  8 ++++++++
 3 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/hvm/vmsi.c b/xen/arch/x86/hvm/vmsi.c
index adbac965f9..9999179837 100644
--- a/xen/arch/x86/hvm/vmsi.c
+++ b/xen/arch/x86/hvm/vmsi.c
@@ -283,8 +283,8 @@ static int msixtbl_write(struct vcpu *v, unsigned long 
address,
     unsigned long flags;
     struct irq_desc *desc;
 
-    if ( (len != 4 && len != 8) || (address & (len - 1)) )
-        return r;
+    if ( !IS_ALIGNED(address, len) )
+        return X86EMUL_OKAY;
 
     rcu_read_lock(&msixtbl_rcu_lock);
 
@@ -345,8 +345,7 @@ static int msixtbl_write(struct vcpu *v, unsigned long 
address,
 
 unlock:
     spin_unlock_irqrestore(&desc->lock, flags);
-    if ( len == 4 )
-        r = X86EMUL_OKAY;
+    r = X86EMUL_OKAY;
 
 out:
     rcu_read_unlock(&msixtbl_rcu_lock);
@@ -357,7 +356,17 @@ static int cf_check _msixtbl_write(
     const struct hvm_io_handler *handler, uint64_t address, uint32_t len,
     uint64_t val)
 {
-    return msixtbl_write(current, address, len, val);
+    /* Ignore invalid length or unaligned writes. */
+    if ( (len != 4 && len != 8) || !IS_ALIGNED(address, len) )
+        return X86EMUL_OKAY;
+
+    /*
+     * This function returns X86EMUL_UNHANDLEABLE even if write is properly
+     * handled, to propagate it to the device model (so it can keep its
+     * internal state in sync).
+     */
+    msixtbl_write(current, address, len, val);
+    return X86EMUL_UNHANDLEABLE;
 }
 
 static bool cf_check msixtbl_range(
diff --git a/xen/common/kernel.c b/xen/common/kernel.c
index 08dbaa2a05..b44b2439ca 100644
--- a/xen/common/kernel.c
+++ b/xen/common/kernel.c
@@ -637,6 +637,7 @@ long do_xen_version(int cmd, XEN_GUEST_HANDLE_PARAM(void) 
arg)
                 fi.submap |= (1U << XENFEAT_hvm_safe_pvclock) |
                              (1U << XENFEAT_hvm_callback_vector) |
                              (has_pirq(d) ? (1U << XENFEAT_hvm_pirqs) : 0);
+            fi.submap |= (1U << XENFEAT_dm_msix_all_writes);
 #endif
             if ( !paging_mode_translate(d) || is_domain_direct_mapped(d) )
                 fi.submap |= (1U << XENFEAT_direct_mapped);
diff --git a/xen/include/public/features.h b/xen/include/public/features.h
index 4437f25d25..8801930947 100644
--- a/xen/include/public/features.h
+++ b/xen/include/public/features.h
@@ -120,6 +120,14 @@
 #define XENFEAT_runstate_phys_area        18
 #define XENFEAT_vcpu_time_phys_area       19
 
+/*
+ * If set, Xen will passthrough all MSI-X vector ctrl writes to device model,
+ * not only those unmasking an entry. This allows device model to properly keep
+ * track of the MSI-X table without having to read it from the device behind
+ * Xen's backs. This information is relevant only for device models.
+ */
+#define XENFEAT_dm_msix_all_writes        20
+
 #define XENFEAT_NR_SUBMAPS 1
 
 #endif /* __XEN_PUBLIC_FEATURES_H__ */
--
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