On Mon, Mar 23, 2020 at 9:39 AM Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote: > > On Mon, Mar 23, 2020 at 05:31:15PM +0100, Alexander Potapenko wrote: > > On Mon, Mar 23, 2020 at 9:18 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > > > > > On 22/03/20 07:59, Dmitry Vyukov wrote: > > > > > > > > The commit range is presumably > > > > fb279f4e238617417b132a550f24c1e86d922558..63849c8f410717eb2e6662f3953ff674727303e7 > > > > But I don't see anything that says "it's me". The only commit that > > > > does non-trivial changes to x86/vmx seems to be "KVM: VMX: check > > > > descriptor table exits on instruction emulation": > > > > > > That seems unlikely, it's a completely different file and it would only > > > affect the outside (non-nested) environment rather than your own kernel. > > > > > > The only instance of "0x86" in the registers is in the flags: > > > > > > > RSP: 0018:ffffc90001ac7998 EFLAGS: 00010086 > > > > RAX: ffffc90001ac79c8 RBX: fffffe0000000000 RCX: 0000000000040000 > > > > RDX: ffffc9000e20f000 RSI: 000000000000b452 RDI: 000000000000b453 > > > > RBP: 0000000000000ec0 R08: ffffffff83987523 R09: ffffffff811c7eca > > > > R10: ffff8880a4e94200 R11: 0000000000000002 R12: dffffc0000000000 > > > > R13: fffffe0000000ec8 R14: ffffffff880016f0 R15: fffffe0000000ecb > > > > FS: 00007fb50e370700(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000 > > > > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > > > > CR2: 000000000000005c CR3: 0000000092fc7000 CR4: 00000000001426f0 > > > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 > > > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 > > > > > > That would suggest a miscompilation of the inline assembly, which does > > > push the flags: > > > > > > #ifdef CONFIG_X86_64 > > > "mov %%" _ASM_SP ", %[sp]\n\t" > > > "and $0xfffffffffffffff0, %%" _ASM_SP "\n\t" > > > "push $%c[ss]\n\t" > > > "push %[sp]\n\t" > > > #endif > > > "pushf\n\t" > > > __ASM_SIZE(push) " $%c[cs]\n\t" > > > CALL_NOSPEC > > > > > > > > > It would not explain why it suddenly started to break, unless the clang > > > version also changed, but it would be easy to ascertain and fix (in > > > either KVM or clang). Dmitry, can you send me the vmx.o and > > > kvm-intel.ko files? > > > > On a quick glance, Clang does not miscompile this part. > > Clang definitely miscompiles the asm, the indirect call operates on the > EFLAGS value, not on @entry as expected. It looks like clang doesn't honor > ASM_CALL_CONSTRAINT, which effectively tells the compiler that %rsp is > getting clobbered, e.g. the "mov %r14,0x8(%rsp)" is loading @entry for > "callq *0x8(%rsp)", which breaks because of asm's pushes. > > clang: > > kvm_before_interrupt(vcpu); > > asm volatile( > ffffffff811b798e: 4c 89 74 24 08 mov %r14,0x8(%rsp) > ffffffff811b7993: 48 89 e0 mov %rsp,%rax > ffffffff811b7996: 48 83 e4 f0 and $0xfffffffffffffff0,%rsp > ffffffff811b799a: 6a 18 pushq $0x18 > ffffffff811b799c: 50 push %rax > ffffffff811b799d: 9c pushfq > ffffffff811b799e: 6a 10 pushq $0x10 > ffffffff811b79a0: ff 54 24 08 callq *0x8(%rsp) <--------- calls the EFLAGS value > kvm_after_interrupt(): > > > gcc: > kvm_before_interrupt(vcpu); > > asm volatile( > ffffffff8118e17c: 48 89 e0 mov %rsp,%rax > ffffffff8118e17f: 48 83 e4 f0 and $0xfffffffffffffff0,%rsp > ffffffff8118e183: 6a 18 pushq $0x18 > ffffffff8118e185: 50 push %rax > ffffffff8118e186: 9c pushfq > ffffffff8118e187: 6a 10 pushq $0x10 > ffffffff8118e189: ff d3 callq *%rbx <-------- calls @entry > kvm_after_interrupt(): Thanks for this analysis, it looks like this is dependent on some particular configuration; here's clang+defconfig+CONFIG_KVM_INTEL=y: 0x000000000000528f <+127>: pushq $0x18 0x0000000000005291 <+129>: push %rcx 0x0000000000005292 <+130>: pushfq 0x0000000000005293 <+131>: pushq $0x10 0x0000000000005295 <+133>: callq *%rax -- Thanks, ~Nick Desaulniers
