linux-kernel - Re: [PATCH 08/26] x86/tdx: Handle in-kernel MMIO

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Message-ID: <20211215233706.6zjqfvpbcx3omszp@black.fi.intel.com>
Date:   Thu, 16 Dec 2021 02:37:06 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...el.com, luto@...nel.org, peterz@...radead.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, knsathya@...nel.org, pbonzini@...hat.com,
        sdeep@...are.com, seanjc@...gle.com, tony.luck@...el.com,
        vkuznets@...hat.com, wanpengli@...cent.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/26] x86/tdx: Handle in-kernel MMIO

On Wed, Dec 15, 2021 at 03:31:16PM -0800, Josh Poimboeuf wrote:
> On Tue, Dec 14, 2021 at 06:02:46PM +0300, Kirill A. Shutemov wrote:
> > @@ -155,6 +157,108 @@ static bool tdx_handle_cpuid(struct pt_regs *regs)
> >  	return true;
> >  }
> >  
> > +static bool tdx_mmio(int size, bool write, unsigned long addr,
> > +		     unsigned long *val)
> > +{
> > +	struct tdx_hypercall_output out;
> > +	u64 err;
> > +
> > +	err = _tdx_hypercall(EXIT_REASON_EPT_VIOLATION, size, write,
> > +			     addr, *val, &out);
> > +	if (err)
> > +		return true;
> > +
> > +	*val = out.r11;
> > +	return false;
> > +}
> > +
> > +static bool tdx_mmio_read(int size, unsigned long addr, unsigned long *val)
> > +{
> > +	return tdx_mmio(size, false, addr, val);
> > +}
> > +
> > +static bool tdx_mmio_write(int size, unsigned long addr, unsigned long *val)
> > +{
> > +	return tdx_mmio(size, true, addr, val);
> > +}
> 
> These bool functions return false on success.  Conversely, other
> functions in this file return true on success.  That inconsistency is
> really confusing for the callers and is bound to introduce bugs
> eventually.
> 
> > +static int tdx_handle_mmio(struct pt_regs *regs, struct ve_info *ve)
> 
> Similarly, tdx_handle_mmio() returns (int) 0 for success, while other
> tdx_handle_*() functions return (bool) true for success.  Also
> confusing.
> 
> The most robust option would be for all the functions to follow the
> typical kernel convention of returning (int) 0 on success.  It works for
> 99.99% of the kernel.  Why mess with success? (pun intended)
> 
> Otherwise it's just pointless added cognitive overhead, trying to keep
> track of what success means, for each individual function.

Okay, fair enough. I will make them consistent.

-- 
 Kirill A. Shutemov