Re: 5.1.0-rc1 regression: reset fails with kvm and -cpu host

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Philippe Mathieu-Daudé <philmd@redhat.com> writes:

> +Vitaly
>
> On 7/23/20 10:40 AM, Dr. David Alan Gilbert wrote:
>> * Eduardo Habkost (ehabkost@redhat.com) wrote:
>>> On Wed, Jul 22, 2020 at 04:47:32PM -0400, Eduardo Habkost wrote:
>>>> On Wed, Jul 22, 2020 at 08:05:01PM +0200, Jan Kiszka wrote:
>>>>> On 22.07.20 19:35, Eduardo Habkost wrote:
>>>>>> Hi Jan,
>>>>>>
>>>>>> What was the last version where it worked for you?  Does using
>>>>>> "-cpu host,-vmx" help?
>>>>>
>>>>> Yeah, -vmx does indeed help.
>>>>>
>>>>> I didn't have the time to bisect yet. Just check my reflog, picked
>>>>> eb6490f544, and that works.
>>>>
>>>> Thanks!
>>>>
>>>> I could reproduce it locally[1], I will bisect it.
>>>>
>>>> The good news is that "-cpu host,+vmx" still works, on commit
>>>> eb6490f544.
>>>>
>>>> [1] Linux 5.6.19-300.fc32.x86_64, Intel Core i7-8665U CPU.
>>>
>>> Bisected to:
>>>
>>> commit b16c0e20c74218f2d69710cedad11da7dd4d2190
>>> Author: Paolo Bonzini <pbonzini@redhat.com>
>>> Date:   Wed May 20 10:49:22 2020 -0400
>>>
>>>     KVM: add support for AMD nested live migration
>>>
>>>     Support for nested guest live migration is part of Linux 5.8, add the
>>>     corresponding code to QEMU.  The migration format consists of a few
>>>     flags, is an opaque 4k blob.
>>>
>>>     The blob is in VMCB format (the control area represents the L1 VMCB
>>>     control fields, the save area represents the pre-vmentry state; KVM does
>>>     not use the host save area since the AMD manual allows that) but QEMU
>>>     does not really care about that.  However, the flags need to be
>>>     copied to hflags/hflags2 and back.
>>>
>>>     In addition, support for retrieving and setting the AMD nested virtualization
>>>     states allows the L1 guest to be reset while running a nested guest, but
>>>     a small bug in CPU reset needs to be fixed for that to work.
>>>
>>>     Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> 
>> Guesswork led me to try reverting the chunk in kvm_put_nested_state;
>> without it the reset seems to work; I can't explain that code though.
>> 

(sorry, missed the beginning of the discussion)

So one does:

(qemu) system_reset 

on Intel wiht '-cpu host' and the result is:

(qemu) KVM: entry failed, hardware error 0x80000021

If you're running a guest on an Intel machine without unrestricted mode
support, the failure can be most likely due to the guest entering an invalid
state for Intel VT. For example, the guest maybe running in big real mode
which is not supported on less recent Intel processors.

EAX=00000064 EBX=91df5efe ECX=00000000 EDX=000003f8
ESI=00000000 EDI=91ee32c0 EBP=90643260 ESP=00013c68
EIP=906428e6 EFL=00000002 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0
ES =0000 00000000 0000ffff 00009300
CS =f000 ffff0000 0000ffff 00009b00
SS =0000 00000000 0000ffff 00009300
DS =0000 00000000 0000ffff 00009300
FS =0000 00000000 0000ffff 00009300
GS =0000 00000000 0000ffff 00009300
LDT=0000 00000000 0000ffff 00008200
TR =0000 00000000 0000ffff 00008b00
GDT=     00000000 0000ffff
IDT=     00000000 0000ffff
CR0=60000010 CR2=00000000 CR3=00000000 CR4=00000000
DR0=0000000000000000 DR1=0000000000000000 DR2=0000000000000000 DR3=0000000000000000 
DR6=00000000ffff0ff0 DR7=0000000000000400
EFER=0000000000000000
Code=?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? <??> ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ??

I can take a look (if no one beats me to it).

-- 
Vitaly