Assertion,'hvmemul_cache_disabled(curr)' failed at vmx.c:4380

[Don Slutz <don.slutz@xxxxxxxxx>]

Currently I only know how to get this assert with debug=y and hvm_fep 
enabled.  The attached test code should reproduce this. This happens 
when any guest does the "ud2 xen" inl that uses QEMU.  Port 0x3f1 is 
what I use to reproduce this.
My understanding is that it is possible to get to the x86 instruction 
emulator for "inl" instruction without hvm_fep enabled. Here is more info:
(XEN) [2020-08-18 12:33:45.880] Assertion 'hvmemul_cache_disabled(curr)' 
failed at vmx.c:4380
(XEN) [2020-08-18 12:33:45.905] ----[ Xen-4.15-unstable  x86_64 
debug=y   Tainted:  C H ]----
(XEN) [2020-08-18 12:33:45.930] CPU:    3
(XEN) [2020-08-18 12:33:45.941] RIP: 
e008:[<ffff82d04029d565>]vmx_vmenter_helper+0x3d/0x45d
(XEN) [2020-08-18 12:33:45.966] RFLAGS: 0000000000010097 CONTEXT: 
hypervisor (d1v0)
...

I did track this to:

commit 2efbc2d446b1315de9c6441a4d535b1fb91e1767
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu Apr 23 09:55:00 2020 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Apr 23 09:55:00 2020 +0200

    x86/HVM: implement memory read caching for insn emulation
...
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Paul Durrant <pdurrant@xxxxxxxx>
...

   -Don Slutz

Attachment: hvm_fep_inl_test.c
Description: Text document