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Date:   Fri, 20 Jan 2023 12:50:51 -0800
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Andrew Cooper <Andrew.Cooper3@citrix.com>,
        Dave Hansen <dave.hansen@intel.com>,
        "Li, Xin3" <xin3.li@intel.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>
CC:     "x86@kernel.org" <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: the x86 sysret_rip test fails on the Intel FRED architecture
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On January 20, 2023 10:52:02 AM PST, Andrew Cooper <Andrew=2ECooper3@citrix=
=2Ecom> wrote:
>On 20/01/2023 5:45 pm, Dave Hansen wrote:
>> On 1/19/23 23:49, Li, Xin3 wrote:
>>> The x86 sysret_rip test has the following assertion:
>>>
>>>         /* R11 and EFLAGS should already match=2E */
>>>         assert(ctx->uc_mcontext=2Egregs[REG_EFL] =3D=3D
>>>                ctx->uc_mcontext=2Egregs[REG_R11]);
>>>
>>> This is being tested to avoid kernel state leak due to sysret vs iret,
>>> but that on FRED r11 is *always* preserved, and the test just fails=2E
>> Let's figure out the reason that FRED acts differently, first=2E  Right
>> now, the SDM says:
>>
>> 	SYSCALL also saves RFLAGS into R11
>>
>> so that behavior of SYSCALL _looks_ architectural to me=2E  Was this
>> change in SYSCALL behavior with FRED intentional?
>
>FRED 3=2E0 Section 7=2E4 says the only changes for the SYSCALL and SYSENT=
ER
>instructions are the enablement conditions=2E=C2=A0 Nowhere else is there
>mention of a FRED OS needing to emulate legacy syscall behaviour by
>adjusting %r11/%rcx
>
>However, ERETU does handle flags different to SYSRET (in particular, I
>think you can establish TF on the instruction boundary after SYSCALL
>now)=2E=C2=A0 What are the raw values of REG_EFL and REG_R11 ?
>
>~Andrew
>

Just to avoid any confusion:

Syscall and sysenter in a FRED system are treated equivalently to software=
 interrupts, e=2Eg=2E INT 0x80=2E They do not modify any registers=2E