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Re: [PATCH] x86emul: Treat the vector of INT imm8 as unsigned
- To: Stephen Cheng <stephen.cheng@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 12 Nov 2026 10:29:49 +0100
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- Cc: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger@xxxxxxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 12 Nov 2026 09:30:01 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 12.11.2026 08:23, Stephen Cheng wrote:
> The decoder sign-extends all 8-bit immediates (SrcImmByte). For INT
> imm8 the immediate is the vector, which is unsigned. So INT $0x80 to
> INT $0xff put a negative vector into the event (e.g. -128 for 0x80).
>
> hvm_inject_event() then fails its "vector == event->vector" assertion,
> and a debug Xen panics. An HVM guest can trigger this with the forced
> emulation prefix (hvm_fep).
>
> Cast the vector to uint8_t, as is already done for the port of IN/OUT
> and the operand of AAM/AAD.
I'm afraid this isn't sufficient: At least x86_insn_immediate() is also
affected, and then also for the other insns you mention above plus quite
a few more (in particular uniformly for 0F, 0F3A and XOP08). I think
this needs sorting in x86_decode_onebyte() for all of the former (some
of which I then need to mirror into the APX series for Map4), and in the
switch() below setting ctxt->opcode and s->desc (where decode_*() are
called) for the latter three.
There'll then want to be a 2nd Fixes: tag for the commit which
introduced x86_insn_immediate().
Jan
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