From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.zytor.com (terminus.zytor.com [198.137.202.136]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6A0B41D88AC; Sat, 26 Jul 2025 04:55:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.137.202.136 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753505723; cv=none; b=My0FK/GxJczw7PlgvuWZC21WrffIjZuYaUWmzes7ynoFLvc1ifYKvpeiFEo8Iv8x0fz+qwKas/UBxuzTDJ3cshlg+Xc6WC4JsA+wgq1WTMs4nCzMm07W10prbcyERjFfQO8KMrJWGAjNRDKvWB1Nhw+57gFc3S/6XRcXpRRPieU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753505723; c=relaxed/simple; bh=WIKJAZbq8ZAJLEZT6znW//XtQXEdPdFm79jVWQWJcpQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=mgcSKIqYOVy+VFay8XEr+NSvzNRnYkV+GEvV3IDgB5+Ed7x8AazkbeMHXJf6wBm3Hxr9WCfS1hzwDAaYEU3z+lHZ7Mh1esZW31eHPsomVfEriNswnVgE3BPbVAeHUN5lp3UYM0tkSG7CQiZXE8x+OrJR+zC/Bqpx0n+s8rN9fQM= ARC-Authentication-Results:i=1; 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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Ibk+EvbwHOgojh0tGX7l4bIpi0ESMCawJY3wF5jFKBklo3JdFibrVjGV0B8bv9FaQ eLpIWFr8UXn+Y7urBwWVJwUAIyfUtJI4D+UBYgB/0rOtyHXXt60emaqIL8yYV/s87M jxZvbK7ppTbcVeKj9Mwag9Zg+kOaz/KSmZeGMC0ZjIfedzO4afO7U0K3PW4xQLxCgG UWSDok1HAtjqWF9uGDygksU6AbFQfBEe9rVYexOKeLV9IsBRitXvsp1xARoZ4PmdST whmUq1ZS4sbmRWH7wkGcWjqilgU2DFj7I+ITt2zMa2GVci9b3EjRvBCkP2JmGwfith SHHzNgZzwXvPg== Message-ID: Date: Fri, 25 Jul 2025 21:54:32 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 14/16] x86/fred: Play nice with invoking asm_fred_entry_from_kvm() on non-FRED hardware To: Peter Zijlstra , x86@kernel.org Cc: kys@microsoft.com, haiyangz@microsoft.com, wei.liu@kernel.org, decui@microsoft.com, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, seanjc@google.com, pbonzini@redhat.com, ardb@kernel.org, kees@kernel.org, Arnd Bergmann , gregkh@linuxfoundation.org, jpoimboe@kernel.org, linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-efi@vger.kernel.org, samitolvanen@google.com, ojeda@kernel.org References: <20250714102011.758008629@infradead.org> <20250714103441.245417052@infradead.org> Content-Language: en-US From: Xin Li Autocrypt: addr=xin@zytor.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 7/14/2025 3:20 AM, Peter Zijlstra wrote: > call __fred_entry_from_kvm /* Call the C entry point */ > - POP_REGS > - ERETS > -1: > + > +1: /* The symbol "1" is misplaced; it needs to be put after the ERETS instruction. > + * When FRED, use ERETS to potentially clear NMIs, otherwise simply > + * restore the stack pointer. > + */ > + ALTERNATIVE "nop; nop; mov %rbp, %rsp", \ Why explicitly add two nops here? ALTERNATIVE will still pad three-byte nop after the MOV instruction. > + __stringify(add $C_PTREGS_SIZE, %rsp; ERETS), \ > + X86_FEATURE_FRED > + > /* > - * Objtool doesn't understand what ERETS does, this hint tells it that > - * yes, we'll reach here and with what stack state. A save/restore pair > - * isn't strictly needed, but it's the simplest form. > + * Objtool doesn't understand ERETS, and the cfi register state is > + * different from initial_func_cfi due to PUSH_REGS. Tell it the state > + * is similar to where UNWIND_HINT_SAVE is. > */ > UNWIND_HINT_RESTORE > + > pop %rbp > RET >