linux-kernel - Re: [PATCH v1 1/1] x86/entry/64: Skip SYSRET validation tests when FRED is enabled

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Message-ID: <ad5bd7f5-3290-4ddc-83bd-41a18fa1beee@zytor.com>
Date: Wed, 10 Apr 2024 09:44:05 -0700
From: Xin Li <xin@...or.com>
To: "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Cc: luto@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, brgerst@...il.com
Subject: Re: [PATCH v1 1/1] x86/entry/64: Skip SYSRET validation tests when
 FRED is enabled

On 4/9/2024 4:35 PM, H. Peter Anvin wrote:
> Now, for a FRED system the return value is ignored anyway, so the only 
> benefit of this is skipping the test, correct?

Yes, and as syscalls are hot paths, we should save some cycles.

> 
> I do observe than what is left of do_fast_syscall_32 is a single 
> assignment followed by __do_fast_syscall_32(), which could be turned 
> into a tailcall.

Ah, I didn't realize this is a better way, will change.

> 
> Another thing that has been added lately is a bunch of tests on the int 
> $0x80 path. This is a slow path *on legacy hardware*, but on FRED 
> systems it is the *fast* path for IA32 emulation. These tests are also 
> totally unnecessary *AND IN FACT, WRONG* on a FRED system, as FRED 
> distinguishes external interrupts from software interrupts.
> 
> Furthermore, under FRED interrupt 0x80 is available as a hardware 
> interrupt (since there is no reason to block it out.) Therefore, going 
> and poking the APIC as in int80_is_external() is INCORRECT and possibly 
> fatal.
> 
> Again, the easiest way to fix that is to follow what XenPV does in 
> int80_is_external(), but there is more unnecessary stuff: 
> !user_mode(regs) cannot happen on FRED, and the stuff in the 
> int80_emulation assembly function should, if it is needed at all on any 
> FRED-compatible hardware (I don't believe so) should be done in the FRED 
> user mode assembly entry stub.
> 
> Thus, it might be better to strip down do_int80_emulation() to a lean 
> fred_int80_emulation().

Yeah, FRED is fundamentally better with its nature.  Will do!

Thanks!
     Xin

> 
>      -hpa
> 
> On 4/2/24 23:24, Xin Li (Intel) wrote:
>> Don't do SYSRET validation tests when FRED is enabled, since ERETU is
>> the only legit instruction to return to user level.
>>
>> Signed-off-by: Xin Li (Intel) <xin@...or.com>
>> ---
>>   arch/x86/entry/common.c | 12 ++++++++++--
>>   1 file changed, 10 insertions(+), 2 deletions(-)