[Qemu-devel] [PATCH] i386: Fix legacy guest with xsave panic on host kvm without update cpuid.

[Bingsong Si <owen.si@ucloud.cn>]

without kvm commit 412a3c41, CPUID(EAX=0xd,ECX=0).EBX always equal to 0 even
through guest update xcr0, this will crash legacy guest(e.g., CentOS 6).
Below is the call trace on the guest.

[    0.000000] kernel BUG at mm/bootmem.c:469!
[    0.000000] invalid opcode: 0000 [#1] SMP
[    0.000000] last sysfs file:
[    0.000000] CPU 0
[    0.000000] Modules linked in:
[    0.000000]
[    0.000000] Pid: 0, comm: swapper Tainted: G           --------------- H  2.6.32-279#2 Red Hat KVM
[    0.000000] RIP: 0010:[<ffffffff81c4edc4>]  [<ffffffff81c4edc4>] alloc_bootmem_core+0x7b/0x29e
[    0.000000] RSP: 0018:ffffffff81a01cd8  EFLAGS: 00010046
[    0.000000] RAX: ffffffff81cb1748 RBX: ffffffff81cb1720 RCX: 0000000001000000
[    0.000000] RDX: 0000000000000040 RSI: 0000000000000000 RDI: ffffffff81cb1720
[    0.000000] RBP: ffffffff81a01d38 R08: 0000000000000000 R09: 0000000000001000
[    0.000000] R10: 02008921da802087 R11: 00000000ffff8800 R12: 0000000000000000
[    0.000000] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000001000000
[    0.000000] FS:  0000000000000000(0000) GS:ffff880002200000(0000) knlGS:0000000000000000
[    0.000000] CS:  0010 DS: 0018 ES: 0018 CR0: 0000000080050033
[    0.000000] CR2: 0000000000000000 CR3: 0000000001a85000 CR4: 00000000001406b0
[    0.000000] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[    0.000000] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[    0.000000] Process swapper (pid: 0, threadinfo ffffffff81a00000, task ffffffff81a8d020)
[    0.000000] Stack:
[    0.000000]  0000000000000002 81a01dd881eaf060 000000007e5fe227 0000000000001001
[    0.000000] <d> 0000000000000040 0000000000000001 0000006cffffffff 0000000001000000
[    0.000000] <d> ffffffff81cb1720 0000000000000000 0000000000000000 0000000000000000
[    0.000000] Call Trace:
[    0.000000]  [<ffffffff81c4f074>] ___alloc_bootmem_nopanic+0x8d/0xca
[    0.000000]  [<ffffffff81c4f0cf>] ___alloc_bootmem+0x11/0x39
[    0.000000]  [<ffffffff81c4f172>] __alloc_bootmem+0xb/0xd
[    0.000000]  [<ffffffff814d42d9>] xsave_cntxt_init+0x249/0x2c0
[    0.000000]  [<ffffffff814e0689>] init_thread_xstate+0x17/0x25
[    0.000000]  [<ffffffff814e0710>] fpu_init+0x79/0xaa
[    0.000000]  [<ffffffff814e27e3>] cpu_init+0x301/0x344
[    0.000000]  [<ffffffff81276395>] ? sort+0x155/0x230
[    0.000000]  [<ffffffff81c30cf2>] trap_init+0x24e/0x25f
[    0.000000]  [<ffffffff81c2bd73>] start_kernel+0x21c/0x430
[    0.000000]  [<ffffffff81c2b33a>] x86_64_start_reservations+0x125/0x129
[    0.000000]  [<ffffffff81c2b438>] x86_64_start_kernel+0xfa/0x109
[    0.000000] Code: 03 48 89 f1 49 c1 e8 0c 48 0f af d0 48 c7 c6 00 a6 61 81 48 c7 c7 00 e5 79 81 31 c0 4c 89 74 24 08 e8 f2 d7 89 ff 4d 85 e4 75 04 <0f> 0b eb fe 48 8b 45 c0 48 83 e8 01 48 85 45
c0 74 04 0f 0b eb

Signed-off-by: Bingsong Si <owen.si@ucloud.cn>
---
 target/i386/cpu.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index ff65e11008..77510cdacd 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -4416,7 +4416,13 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
             *ecx = xsave_area_size(x86_cpu_xsave_components(cpu));
             *eax = env->features[FEAT_XSAVE_COMP_LO];
             *edx = env->features[FEAT_XSAVE_COMP_HI];
-            *ebx = xsave_area_size(env->xcr0);
+            /*
+             * The initial value of xcr0 and ebx == 0, On host without kvm
+             * commit 412a3c41(e.g., CentOS 6), the ebx's value always == 0
+             * even through guest update xcr0, this will crash some legacy guest
+             * (e.g., CentOS 6), So set ebx == ecx to workaroud it.
+             */
+            *ebx = kvm_enabled() ? *ecx : xsave_area_size(env->xcr0);
         } else if (count == 1) {
             *eax = env->features[FEAT_XSAVE];
         } else if (count < ARRAY_SIZE(x86_ext_save_areas)) {
-- 
2.22.0