Re: [Xen-devel] [PATCH v2] SVM: Add union intstat_t for offset 68h in vmcb struct

[Pu Wen <puwen@hygon.cn>]

On 2020/3/24 20:28, Andrew Cooper wrote:
> Hmm - this field doesn't appear to be part of AVIC, which makes me
> wonder what we're doing without it.
> 
> It appears to be a shadow copy of EFLAGS.IF which is only written on
> vmexit, and never consumed, but this is based on Appendix B which is the
> only reference I can find to the field at all.  Neither the
> VMRUN/#VMEXIT descriptions discuss it at all.
> 
> Given its position next to the (ambiguous) INTERRUPT_SHADOW, it just
> might actually distinguish the STI shadow from the MovSS shadow, but it
> could only do that by not behaving as described, and being asymmetric
> with EFLAGS.  I don't have time to investigate this right now.
> 
> We need the field described in Xen to set it appropriately for virtual
> vmexit, but I think that is the extent of what we need to do.

We encountered problem while running xen with new firmware which
implement the bit[1] of the VMCB offset 68h: the DomU stopped when
running seabios. We debugged the seabios code and found that the
seabios hung after call16_int10().

Then we debugged the xen code, and found the cause at this place in
svm_get_interrupt_shadow():
    if ( vmcb->interrupt_shadow )
         intr_shadow |= HVM_INTR_SHADOW_MOV_SS | HVM_INTR_SHADOW_STI;
the conditional is true if bit[1] is 1 even though bit[0] is zero.
If just only use bit[0] in the conditional, the problem is solved, DomU
will run successfully.

-- 
Regards,
Pu Wen