On Thu Mar 12, 2026 at 12:42 PM CET, Jan Beulich wrote:
> On 12.03.2026 12:21, Alejandro Vallejo wrote:
>> --- a/xen/arch/x86/hvm/hvm.c
>> +++ b/xen/arch/x86/hvm/hvm.c
>> @@ -3837,35 +3837,34 @@ void hvm_ud_intercept(struct cpu_user_regs *regs)
>> struct vcpu *cur = current;
>> bool should_emulate = false;
>> struct hvm_emulate_ctxt ctxt;
>> + const struct segment_register *cs;
>> + uint32_t walk;
>> + unsigned long addr;
>> + char sig[5]; /* ud2; .ascii "xen" */
>>
>> hvm_emulate_init_once(&ctxt, NULL, regs);
>>
>> - if ( opt_hvm_fep )
>> + cs = &ctxt.seg_reg[x86_seg_cs];
>> + walk = ((ctxt.seg_reg[x86_seg_ss].dpl == 3)
>> + ? PFEC_user_mode : 0) | PFEC_insn_fetch;
>
> While of course functionally everything's fine this way, I'm now entirely
> lost:
> Why are what were ...
>
>> + if ( hvm_virtual_to_linear_addr(x86_seg_cs, cs, regs->rip,
>> + sizeof(sig), hvm_access_insn_fetch,
>> + cs, &addr) &&
>> + (hvm_copy_from_guest_linear(sig, addr, sizeof(sig),
>> + walk, NULL) == HVMTRANS_okay) &&
>> + (memcmp(sig, "\xf\xb" "xen", sizeof(sig)) == 0) )
>> {
>> - const struct segment_register *cs = &ctxt.seg_reg[x86_seg_cs];
>> - uint32_t walk = ((ctxt.seg_reg[x86_seg_ss].dpl == 3)
>> - ? PFEC_user_mode : 0) | PFEC_insn_fetch;
>
> ... initializers before not initializers anymore, when all you're doing is
> (supposedly) re-indentation (and, necessarily, moving decls up to the top of
> the scope they need to live in)?
walk cannot be initialised at the delcaration site because it relies on
hvm_emulate_init_once() having run and you made a strong argument to preserve
the current form so I left cs next to walk, as it was before.
>
>> - unsigned long addr;
>> - char sig[5]; /* ud2; .ascii "xen" */
>> -
>> - if ( hvm_virtual_to_linear_addr(x86_seg_cs, cs, regs->rip,
>> - sizeof(sig), hvm_access_insn_fetch,
>> - cs, &addr) &&
>> - (hvm_copy_from_guest_linear(sig, addr, sizeof(sig),
>> - walk, NULL) == HVMTRANS_okay) &&
>> - (memcmp(sig, "\xf\xb" "xen", sizeof(sig)) == 0) )
>> - {
>> - regs->rip += sizeof(sig);
>> - regs->eflags &= ~X86_EFLAGS_RF;
>> + regs->rip += sizeof(sig);
>> + regs->eflags &= ~X86_EFLAGS_RF;
>>
>> - /* Zero the upper 32 bits of %rip if not in 64bit mode. */
>> - if ( !(hvm_long_mode_active(cur) && cs->l) )
>> - regs->rip = (uint32_t)regs->rip;
>> + /* Zero the upper 32 bits of %rip if not in 64bit mode. */
>> + if ( !(hvm_long_mode_active(cur) && cs->l) )
>> + regs->rip = (uint32_t)regs->rip;
>>
>> - add_taint(TAINT_HVM_FEP);
>> + add_taint(TAINT_HVM_FEP);
>>
>> - should_emulate = true;
>> - }
>> + should_emulate = true;
>> }
>>
>> if ( !should_emulate )
>
> With this, the purpose of the should_emulate variable effectively vanishes,
> without it actually being purged (unlike you had it earlier).
That goes away with patch 6, that I neglected to send before.
>
> Jan
