Re: [PATCH v3] x86/HVM: more consistently set I/O completion

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.08.2020 09:09, Jan Beulich wrote:
> Doing this just in hvm_emulate_one_insn() is not enough.
> hvm_ud_intercept() and hvm_emulate_one_vm_event() can get invoked for
> insns requiring one or more continuations, and at least in principle
> hvm_emulate_one_mmio() could, too. Without proper setting of the field,
> handle_hvm_io_completion() will do nothing completion-wise, and in
> particular the missing re-invocation of the insn emulation paths will
> lead to emulation caching not getting disabled in due course, causing
> the ASSERT() in {svm,vmx}_vmenter_helper() to trigger.
> 
> Reported-by: Don Slutz <don.slutz@xxxxxxxxx>
> 
> Similar considerations go for the clearing of vio->mmio_access, which
> gets moved as well.
> 
> Additionally all updating of vio->mmio_* now gets done dependent upon
> the new completion value, rather than hvm_ioreq_needs_completion()'s
> return value. This is because it is the completion chosen which controls
> what path will be taken when handling the completion, not the simple
> boolean return value. In particular, PIO completion doesn't involve
> going through the insn emulator, and hence emulator state ought to get
> cleared early (or it won't get cleared at all).
> 
> The new logic, besides allowing for a caller override for the
> continuation type to be set (for VMX real mode emulation), will also
> avoid setting an MMIO completion when a simpler PIO one will do. This
> is a minor optimization only as a side effect - the behavior is strictly
> needed at least for hvm_ud_intercept(), as only memory accesses can
> successfully complete through handle_mmio(). Care of course needs to be
> taken to correctly deal with "mixed" insns (doing both MMIO and PIO at
> the same time, i.e. INS/OUTS). For this, hvmemul_validate() now latches
> whether the insn being emulated is a memory access, as this information
> is no longer easily available at the point where we want to consume it.
> 
> Note that the presence of non-NULL .validate fields in the two ops
> structures in hvm_emulate_one_mmio() was really necessary even before
> the changes here: Without this, passing non-NULL as middle argument to
> hvm_emulate_init_once() is meaningless.
> 
> The restrictions on when the #UD intercept gets actually enabled are why
> it was decided that this is not a security issue:
> - the "hvm_fep" option to enable its use is a debugging option only,
> - for the cross-vendor case is considered experimental, even if
>   unfortunately SUPPORT.md doesn't have an explicit statement about
>   this.
> The other two affected functions are
> - hvm_emulate_one_vm_event(), used for introspection,
> - hvm_emulate_one_mmio(), used for Dom0 only,
> which aren't qualifying this as needing an XSA either.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Tested-by: Don Slutz <don.slutz@xxxxxxxxx>
> ---
> v3: Add comment ahead of _hvm_emulate_one(). Add parentheses in a
>     conditional expr. Justify why this does not need an XSA.
> v2: Make updating of vio->mmio_* fields fully driven by the new
>     completion value.
> ---
> I further think that the entire tail of _hvm_emulate_one() (everything
> past the code changed/added there by this patch) wants skipping in case
> a completion is needed, at the very least for the mmio and realmode
> cases, where we know we'll come back here.

Does one of the two of you have an opinion on this aspect?

Jan