[PULL 0/2] Bugfixes for QEMU 7.0-rc1

[PULL 0/2] Bugfixes for QEMU 7.0-rc1

[Paolo Bonzini]

The following changes since commit e2fb7d8aa218256793df99571d16f92074258447:

  Merge tag 'dbus-pull-request' of gitlab.com:marcandre.lureau/qemu into staging (2022-03-15 16:28:50 +0000)

are available in the Git repository at:

  https://gitlab.com/bonzini/qemu.git tags/for-upstream

for you to fetch changes up to 17e6ffa6a5d2674cb2ebfd967d28b1048261d977:

  hw/i386/amd_iommu: Fix maybe-uninitialized error with GCC 12 (2022-03-21 15:57:47 +0100)

----------------------------------------------------------------
Bugfixes.

----------------------------------------------------------------
Paolo Bonzini (2):
      target/i386: kvm: do not access uninitialized variable on older kernels
      hw/i386/amd_iommu: Fix maybe-uninitialized error with GCC 12

 hw/i386/amd_iommu.c   |  7 ++-----
 target/i386/kvm/kvm.c | 17 +++++++++++++----
 2 files changed, 15 insertions(+), 9 deletions(-)
-- 
2.35.1

[PULL 1/2] target/i386: kvm: do not access uninitialized variable on older kernels

[Paolo Bonzini]

KVM support for AMX includes a new system attribute, KVM_X86_XCOMP_GUEST_SUPP.
Commit 19db68ca68 ("x86: Grant AMX permission for guest", 2022-03-15) however
did not fully consider the behavior on older kernels.  First, it warns
too aggressively.  Second, it invokes the KVM_GET_DEVICE_ATTR ioctl
unconditionally and then uses the "bitmask" variable, which remains
uninitialized if the ioctl fails.  Third, kvm_ioctl returns -errno rather
than -1 on errors.

While at it, explain why the ioctl is needed and KVM_GET_SUPPORTED_CPUID
is not enough.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 target/i386/kvm/kvm.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index ef2c68a6f4..06901c2a43 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -411,6 +411,12 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, uint32_t function,
         }
     } else if (function == 0xd && index == 0 &&
                (reg == R_EAX || reg == R_EDX)) {
+        /*
+         * The value returned by KVM_GET_SUPPORTED_CPUID does not include
+         * features that still have to be enabled with the arch_prctl
+         * system call.  QEMU needs the full value, which is retrieved
+         * with KVM_GET_DEVICE_ATTR.
+         */
         struct kvm_device_attr attr = {
             .group = 0,
             .attr = KVM_X86_XCOMP_GUEST_SUPP,
@@ -419,13 +425,16 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, uint32_t function,
 
         bool sys_attr = kvm_check_extension(s, KVM_CAP_SYS_ATTRIBUTES);
         if (!sys_attr) {
-            warn_report("cannot get sys attribute capabilities %d", sys_attr);
+            return ret;
         }
 
         int rc = kvm_ioctl(s, KVM_GET_DEVICE_ATTR, &attr);
-        if (rc == -1 && (errno == ENXIO || errno == EINVAL)) {
-            warn_report("KVM_GET_DEVICE_ATTR(0, KVM_X86_XCOMP_GUEST_SUPP) "
-                        "error: %d", rc);
+        if (rc < 0) {
+            if (rc != -ENXIO) {
+                warn_report("KVM_GET_DEVICE_ATTR(0, KVM_X86_XCOMP_GUEST_SUPP) "
+                            "error: %d", rc);
+            }
+            return ret;
         }
         ret = (reg == R_EAX) ? bitmask : bitmask >> 32;
     } else if (function == 0x80000001 && reg == R_ECX) {
-- 
2.35.1

[PULL 2/2] hw/i386/amd_iommu: Fix maybe-uninitialized error with GCC 12

[Paolo Bonzini]

Be more explicit that the loop must roll at least once.  Avoids the
following warning:

  FAILED: libqemu-x86_64-softmmu.fa.p/hw_i386_amd_iommu.c.o
  In function 'pte_get_page_mask',
      inlined from 'amdvi_page_walk' at hw/i386/amd_iommu.c:945:25,
      inlined from 'amdvi_do_translate' at hw/i386/amd_iommu.c:989:5,
      inlined from 'amdvi_translate' at hw/i386/amd_iommu.c:1038:5:
  hw/i386/amd_iommu.c:877:38: error: 'oldlevel' may be used uninitialized [-Werror=maybe-uninitialized]
    877 |     return ~((1UL << ((oldlevel * 9) + 3)) - 1);
        |                      ~~~~~~~~~~~~~~~~^~~~
  hw/i386/amd_iommu.c: In function 'amdvi_translate':
  hw/i386/amd_iommu.c:906:41: note: 'oldlevel' was declared here
    906 |     unsigned level, present, pte_perms, oldlevel;
        |                                         ^~~~~~~~
  cc1: all warnings being treated as errors

Having:

  $ gcc --version
  gcc (Debian 12-20220313-1) 12.0.1 20220314 (experimental)

Reported-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 hw/i386/amd_iommu.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/hw/i386/amd_iommu.c b/hw/i386/amd_iommu.c
index 4d13d8e697..6986ad3b87 100644
--- a/hw/i386/amd_iommu.c
+++ b/hw/i386/amd_iommu.c
@@ -913,7 +913,7 @@ static void amdvi_page_walk(AMDVIAddressSpace *as, uint64_t *dte,
         }
 
         /* we are at the leaf page table or page table encodes a huge page */
-        while (level > 0) {
+        do {
             pte_perms = amdvi_get_perms(pte);
             present = pte & 1;
             if (!present || perms != (perms & pte_perms)) {
@@ -932,10 +932,7 @@ static void amdvi_page_walk(AMDVIAddressSpace *as, uint64_t *dte,
             }
             oldlevel = level;
             level = get_pte_translation_mode(pte);
-            if (level == 0x7) {
-                break;
-            }
-        }
+        } while (level > 0 && level < 7);
 
         if (level == 0x7) {
             page_mask = pte_override_page_mask(pte);
-- 
2.35.1

Re: [PULL 0/2] Bugfixes for QEMU 7.0-rc1

[Peter Maydell]

On Mon, 21 Mar 2022 at 16:48, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> The following changes since commit e2fb7d8aa218256793df99571d16f92074258447:
>
>   Merge tag 'dbus-pull-request' of gitlab.com:marcandre.lureau/qemu into staging (2022-03-15 16:28:50 +0000)
>
> are available in the Git repository at:
>
>   https://gitlab.com/bonzini/qemu.git tags/for-upstream
>
> for you to fetch changes up to 17e6ffa6a5d2674cb2ebfd967d28b1048261d977:
>
>   hw/i386/amd_iommu: Fix maybe-uninitialized error with GCC 12 (2022-03-21 15:57:47 +0100)
>
> ----------------------------------------------------------------
> Bugfixes.
>
> ----------------------------------------------------------------
> Paolo Bonzini (2):
>       target/i386: kvm: do not access uninitialized variable on older kernels
>       hw/i386/amd_iommu: Fix maybe-uninitialized error with GCC 12


Applied, thanks.

Please update the changelog at https://wiki.qemu.org/ChangeLog/7.0
for any user-visible changes.

-- PMM