linux-kernel - RE: [PATCH v3 6/6] KVM: x86: allow defining return-0 static calls

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Message-ID: <a63028d4ece24040b382ab2bba50bcad@AcuMS.aculab.com>
Date:   Thu, 17 Mar 2022 22:30:39 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Maxim Levitsky' <mlevitsk@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     "seanjc@...gle.com" <seanjc@...gle.com>
Subject: RE: [PATCH v3 6/6] KVM: x86: allow defining return-0 static calls

> I guess something with returning u64 made 32 compiler unhappy.
> 
> I suspect that __static_call_return0 returns long, which means it returns
> 32 bit value on 32 bit system.
> 
> Maybe we can make it return u64. I don't know if that will break something else though.

32 bit system will return a 64bit value in %edx:%eax.
Since %rdx is a scratch register (caller saved) it is fine.

Is __statc_call_return0() ever used to return a NULL pointer?
ISTR some ABI (maybe m68k ones) that returned pointers and
integers in different registers.
Plausibly that was only 35 years ago.

	David

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