Re: [PATCH 07/21] KVM: VMX: Introduce test mode related to EPT violation VE

[Huang, Kai]

On Tue, 2024-03-12 at 09:54 -0700, Sean Christopherson wrote:
> On Tue, Mar 12, 2024, Kai Huang wrote:
> > On 28/02/2024 12:20 pm, Paolo Bonzini wrote:
> > > From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> > > 
> > > To support TDX, KVM is enhanced to operate with #VE.  For TDX, KVM uses the
> > > suppress #VE bit in EPT entries selectively, in order to be able to trap
> > > non-present conditions.  However, #VE isn't used for VMX and it's a bug
> > > if it happens.  To be defensive and test that VMX case isn't broken
> > > introduce an option ept_violation_ve_test and when it's set, BUG the vm.
> > 
> > I am wondering from HW's point of view, is it OK for the kernel to
> > explicitly send #VE IPI, in which case, IIUC, the guest can legally get the
> > #VE w/o being a TDX guest?
> 
> Ooh, fun.  Short answer: there's nothing to worry about here.
> 
> Legally, no.  Vectors 0-31 are reserved.  However, I do _think_ the guest could
> technically send IPIs on vectors 16-31, as the local APIC doesn't outright reject
> such vectors.  But such software would be in clear violation of the SDM.
> 
>   11.5.2 Valid Interrupt Vectors
>   
>   The Intel 64 and IA-32 architectures define 256 vector numbers, ranging from
>   0 through 255 (see Section 6.2, “Exception and Interrupt Vectors”). Local and
>   I/O APICs support 240 of these vectors (in the range of 16 to 255) as valid
>   interrupts.
>   
>   When an interrupt vector in the range of 0 to 15 is sent or received through
>   the local APIC, the APIC indicates an illegal vector in its Error Status
>   Register (see Section 11.5.3, “Error Handling”). The Intel 64 and IA-32
>   architectures reserve vectors 16 through 31 for predefined interrupts,
>   exceptions, and Intel-reserved encodings (see Table 6-1). However, the local
>   APIC does not treat vectors in this range as illegal.
> 
>   When an illegal vector value (0 to 15) is written to an LVT entry and the delivery
>   mode is Fixed (bits 8-11 equal 0), the APIC may signal an illegal vector error,
>   without regard to whether the mask bit is set or whether an interrupt is actually
>   seen on the input.

I hate the "may" here :-)

> 
> where Table 6-1 defines the various exceptions, including #VE, and for vectors
> 22-31 says "Intel reserved. Do not use."  Vectors 32-255 are explicitly described
> as "User Defined (Non-reserved) Interrupts" that can be generated via "External
> interrupt or INT n instruction."
> 
> However, INTn is far more interesting than IPIs, as INTn can definitely generate
> interrupts for vectors 0-31, and the legality of software generating such interrupts
> is questionable.  E.g. KVM used to "forward" NMI VM-Exits to the kernel by doing
> INTn with vector 2.
> 
> Key word "interrupts"!  IPIs are hardware interrupts, and INTn generates software
> interrupts, neither of which are subject to exception bitmap interception:
> 
>   Exceptions (faults, traps, and aborts) cause VM exits based on the exception
>   bitmap (see Section 25.6.3). If an exception occurs, its vector (in the range
>   0–31) is used to select a bit in the exception bitmap. If the bit is 1, a VM
>   exit occurs; if the bit is 0, the exception is delivered normally through the
>   guest IDT. This use of the exception bitmap applies also to exceptions generated
>   by the instructions INT1, INT3, INTO, BOUND, UD0, UD1, and UD2.
> 
> with a footnote that further says:
> 
>   INT1 and INT3 refer to the instructions with opcodes F1 and CC, respectively,
>   and not to INT n with value 1 or 3 for n.
> 
> So while a misbehaving guest could generate a software interrupt on vector 20,
> it would not be a true #VE, i.e. not an exception, and thus would not generate
> an EXCEPTION_NMI VM-Exit.  I.e. the KVM_BUG_ON() can't be triggered by the guest
> (assuming hardware isn't broken).
> 

Ah, right, software-interrupts but not exceptions.  

Thanks for the full explanation!