Messages in this thread | | | From | stsp <> | Subject | KVM_SET_CPUID doesn't check supported bits (was Re: [PATCH 0/6] KVM: x86: KVM_SET_SREGS.CR4 bug fixes and cleanup) | Date | Mon, 7 Dec 2020 14:24:58 +0300 |
| |
[re-send because of bad formatting]
09.10.2020 18:30, Sean Christopherson пишет: >> The only other effect of setting VMXE was clearing VME. Which shouldn't >> affect anything either, right? > Hmm, clearing VME would mean that exceptions/interrupts within the > guest would > trigger a switch out of v86 and into vanilla protected mode. v86 and > PM have > different consistency checks, particularly for segmentation, so it's > plausible > that clearing CR4.VME inadvertantly worked around the bug by avoiding > invalid > guest state for v86.
Almost.
So with your patch set (thanks!) and a bit of further investigations, it now became clear where the problem is. We have this code: ---
|cpuid->nent = 2; // Use the same values as in emu-i386/simx86/interp.c // (Pentium 133-200MHz, "GenuineIntel") cpuid->entries[0] = (struct kvm_cpuid_entry) { .function = 0, .eax = 1, .ebx = 0x756e6547, .ecx = 0x6c65746e, .edx = 0x49656e69 }; // family 5, model 2, stepping 12, fpu vme de pse tsc msr mce cx8 cpuid->entries[1] = (struct kvm_cpuid_entry) { .function = 1, .eax = 0x052c, .ebx = 0, .ecx = 0, .edx = 0x1bf }; ret = ioctl(vcpufd, KVM_SET_CPUID, cpuid); free(cpuid); if (ret == -1) { perror("KVM: KVM_SET_CPUID"); return 0; }|
---
It tries to enable VME among other things. qemu appears to disable VME by default, unless you do "-cpu host". So we have a situation where the host (which is qemu) doesn't have VME, and guest (dosemu) is trying to enable it. Now obviously KVM_SET_CPUID doesn't check anyting at all and returns success. That later turns into an invalid guest state.
Question: should KVM_SET_CPUID check for supported bits, end return error if not everything is supported?
| |