Re: [PATCH v5 2/3] x86/insn-eval: Add insn_assign_reg() helper

[Sean Christopherson]

On Thu, Jul 02, 2026, Kiryl Shutsemau wrote:
> On Wed, Jul 01, 2026 at 06:00:33PM +0100, David Laight wrote:
> > Or be even more specific and use '& 0xffffffff' rather than a cast.
> > Particularly since the casts of the RHS in the byte/short cases aren't
> > needed at all.
> 
> I'd rather keep the body exactly as KVM has it today.

+1.  My main argument for casting in the 1-byte and 2-byte cases is consistency
above all else, using a mask for the 4-byte case defeats that goal.

> This is now a straight move + rename with no functional change, and the v4
> attempt to rewrite it with arithmetic is precisely what introduced the
> AH/CH/DH/BH clobber Sashiko flagged.  Tidying the casts turns it back into a
> rewrite and diverges from the form KVM has shipped for years.  Feel free to
> submit a separate cleanup on top if you feel strongly.
> 
> Updated patch below; I'll fold it into v6.
> 
> -- >8 --
> Subject: [PATCH] x86/insn-eval: Move assign_register() out of KVM as insn_assign_reg()
> 
> KVM's instruction emulator has a small helper, assign_register(), that
> writes a value into a register following the x86 rules for writes to
> general-purpose registers: an 8- or 16-bit write leaves the rest of the
> register untouched, a 32-bit write zero-extends the result to 64 bits,
> and a 64-bit write replaces the whole register.
> 
> The TDX guest #VE handler needs the same logic for port I/O emulation
> to get 32-bit zero-extension right.  Rather than add a third copy of
> the same switch, move the helper verbatim to <asm/insn-eval.h>, rename
> it to insn_assign_reg(), and route KVM's callers through it.
> 
> Add <asm/insn.h> to the header's includes so it builds standalone in
> callers that have not pulled it in transitively.
> 
> No functional change.
> 
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # prerequisite for the following 32-bit port I/O zero-extension fix
> ---

Acked-by: Sean Christopherson <seanjc@xxxxxxxxxx>