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Re: [RFC v1 3/7] x86/emul: Separate out instruction completion
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
- Date: Thu, 21 May 2026 10:59:32 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 21 May 2026 09:59:49 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 5/19/26 11:08 AM, Jan Beulich wrote:
On 18.05.2026 15:14, Ross Lagerwall wrote:
To support merging the emulated instruction and VMEXIT paths, split out
instruction completion from the core x86 emulation so it can be called
separately.
You don't mean to use a full-fledged struct x86_emulate_ctxt on the VMEXIT
paths, do you?
For VMEXITs it uses a struct hvm_emulate_ctxt which contains struct
x86_emulate_ctxt but without going through the full emulator. See patch 4 for
details (probably makes sense to discuss this in the context of that patch).
@@ -1265,8 +1281,6 @@ x86_emulate(
uint8_t b, d, *opc = NULL;
unsigned int first_byte = 0, elem_bytes, insn_bytes = 0;
uint64_t op_mask = ~0ULL;
- bool singlestep = (_regs.eflags & X86_EFLAGS_TF) &&
- !is_branch_step(ctxt, ops);
Nit: Yes, indentation was screwed here. Please ...
@@ -1280,6 +1294,9 @@ x86_emulate(
init_context(ctxt);
+ ctxt->singlestep = (_regs.eflags & X86_EFLAGS_TF) &&
+ !is_branch_step(ctxt, ops);
... correct such when moving code around.
OK.
@@ -8347,17 +8364,6 @@ x86_emulate(
put_fpu(fpu_type, false, state, ctxt, ops);
fpu_type = X86EMUL_FPU_none;
- /* Zero the upper 32 bits of %rip if not in 64-bit mode. */
- if ( !mode_64bit() )
- _regs.r(ip) = (uint32_t)_regs.r(ip);
While, because you have the new helper update ctxt->regs, the removal of
this update of _regs looks technically okay, ...
- /* Should a singlestep #DB be raised? */
- if ( rc == X86EMUL_OKAY && singlestep && !ctxt->retire.mov_ss )
- {
- ctxt->retire.singlestep = true;
- ctxt->retire.sti = false;
- }
-
if ( rc != X86EMUL_DONE )
*ctxt->regs = _regs;
... further uses of _regs here (not this one, but in general) would be at
risk of no longer working as expected (yet only in a corner case, so
possibly not covered by testing).
Any uses of _regs after assignment back to the context is surely broken anyway
because nothing outside of this function would see the result? Not that I would
expect that to happen anyway since this is essentially the end of the function.
Ross
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