linux-kernel - Re: [PATCH 01/15] arm64: kvm: Unify users of HVC instruction

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Message-ID: <20200513103041.ktaov2jws72itmag@dbrazdil-macbookpro.roam.corp.google.com>
Date:   Wed, 13 May 2020 11:30:41 +0100
From:   David Brazdil <dbrazdil@...gle.com>
To:     Quentin Perret <qperret@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, David Brazdil <dbrazdil@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Will Deacon <will@...nel.org>, kvmarm@...ts.cs.columbia.edu,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/15] arm64: kvm: Unify users of HVC instruction

On Thu, May 07, 2020 at 03:36:33PM +0100, Quentin Perret wrote:
> On Thursday 07 May 2020 at 15:01:07 (+0100), Marc Zyngier wrote:
> > >  /*
> > > - * u64 __kvm_call_hyp(void *hypfn, ...);
> > > + * u64 __kvm_call_hyp(unsigned long arg, ...);
> > >   *
> > >   * This is not really a variadic function in the classic C-way and care must
> > >   * be taken when calling this to ensure parameters are passed in registers
> > >   * only, since the stack will change between the caller and the callee.
> > > - *
> > > - * Call the function with the first argument containing a pointer to the
> > > - * function you wish to call in Hyp mode, and subsequent arguments will be
> > > - * passed as x0, x1, and x2 (a maximum of 3 arguments in addition to the
> > > - * function pointer can be passed).  The function being called must be mapped
> > > - * in Hyp mode (see init_hyp_mode in arch/arm/kvm/arm.c).  Return values are
> > > - * passed in x0.
> > > - *
> > > - * A function pointer with a value less than 0xfff has a special meaning,
> > > - * and is used to implement hyp stubs in the same way as in
> > > - * arch/arm64/kernel/hyp_stub.S.
> > 
> > I don't think any of this becomes obsolete with this patch (apart from
> > the reference to 32bit), and only changes with patch #2. Or am I
> > misunderstanding something?
> 
> Nope, I think you're right. To be fair, this patch has changed quite
> a bit since the first version (which did change that comment a little
> later IIRC), but David has done all the hard work on top so I'll let
> him answer that one.
They have a different meaning in the three different contexts:
	1) hyp-stub HVC: hcall ID + 3 args
	2) hyp-init HVC: 4 args (no hcall ID)
	3) host HVC: function  pointer + 3 args
The patch was meant to have all three use the same HVC routine, eventually
converging on host HVCs also using 'hcall ID + 3 args' in patch 02, but it is
a bit forced at this point. I can drop this patch from the series and we can
clean this up later if it still makes sense.

> 
> And David, feel free to take the authorship for this patch -- I barely
> recognize it (for the better), so it's more than fair if you get the
> credit :)