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From: Dave Hansen <dave.hansen@intel.com>
To: "Kiryl Shutsemau (Meta)" <kas@kernel.org>,
	Thomas Gleixner <tglx@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org
Cc: "H . Peter Anvin" <hpa@zytor.com>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Kuppuswamy Sathyanarayanan
	<sathyanarayanan.kuppuswamy@linux.intel.com>,
	Kai Huang <kai.huang@intel.com>,
	Borys Tsyrulnikov <tsyrulnikov.borys@gmail.com>,
	linux-kernel@vger.kernel.org, linux-coco@lists.linux.dev,
	kvm@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH v2 2/2] x86/tdx: Fix zero-extension for 32-bit port I/O
Date: Tue, 12 May 2026 18:14:13 -0700	[thread overview]
Message-ID: <bf92ebbf-8d70-406a-aea1-c11ca576de90@intel.com> (raw)
In-Reply-To: <20260428125632.129770-3-kas@kernel.org>

On 4/28/26 05:56, Kiryl Shutsemau (Meta) wrote:
> +	if (size == 4)
> +		regs->ax = 0;
> +	else
> +		regs->ax &= ~mask;

I haven't thought about this _that_ much, but this feels wrong. Why is
is 4 so special cased?

Also, what _are_ the limits on the registers that 'in' can be used on?

RAX - n/a, no 64-bit I/O
EAX - size=4
AX  - size=2
AH  - n/a no encoding for inb
AL  - size=1

I'd find this much easier to grasp if there was a nice table of what the
registers, sizes, and masks ended up being usable. As usual, x86 is
"fun" here.

  parent reply	other threads:[~2026-05-13  1:14 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20260428125632.129770-1-kas@kernel.org>
2026-05-08 22:52 ` [PATCH v2 0/2] x86/tdx: Port I/O emulation fixes Kiryl Shutsemau
2026-05-08 22:53   ` Dave Hansen
     [not found] ` <20260428125632.129770-3-kas@kernel.org>
2026-05-13  1:14   ` Dave Hansen [this message]
2026-05-13  2:32     ` [PATCH v2 2/2] x86/tdx: Fix zero-extension for 32-bit port I/O H. Peter Anvin
2026-05-13 12:48       ` Sean Christopherson

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