linux-kernel - Re: [PATCH v5 04/17] x86/acrn: Introduce hypercall interfaces

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Date:   Mon, 2 Nov 2020 14:01:13 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     shuo.a.liu@...el.com, linux-kernel@...r.kernel.org, x86@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Yu Wang <yu1.wang@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Yakui Zhao <yakui.zhao@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Fengwei Yin <fengwei.yin@...el.com>,
        Zhi Wang <zhi.a.wang@...el.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Peter Zijlstra <peterz@...radead.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v5 04/17] x86/acrn: Introduce hypercall interfaces

On Mon, Nov 02, 2020 at 07:34:30PM +0100, Borislav Petkov wrote:
> On Mon, Nov 02, 2020 at 12:10:00PM -0600, Segher Boessenkool wrote:
> > (It does not allow *all* memory and *all* constants, btw...  And the
> > condition for registers is not really "general register", whatever that
> > means...
> 
> I think that means general purpose registers. I.e., it won't use, say
> FPU, XMM or whatever special regs.
> 
> What does the asm() parsing code in gcc do for "g"? There should be
> some logic which constraints what register is valid...

It just asks the general_operand function, which (for registers) accepts
the hard registers that are accessible.  This does include the float and
vector etc. registers, normally.

But you usually have a pseudo-register there (which is always allowed
here), unless you used some register asm variable.  And pseudos usually
are allocated a simple integer register during register allocation, in
an asm that is.

> > I hope no one ever told you our documentation does not have white
> > lies!)
> 
> I have convinced myself of this a couple of times already so I either go
> ask our gcc friends or go look straight at gcc source. It is useful to
> know folks which hack on it so that I can ask them stupid questions and
> not go off into the weeds, trying to figure out what the documentation
> says.
> 
> But hey, if that gets the documentation improved, that's a win-win
> situation right there.

Yes :-)

> Might even make people copying from bad examples
> to go look at the docs first...

Optimism is cool :-)


Segher