linux-kernel - Re: [PATCH v11 11/20] x86/virt/tdx: Fill out TDMRs to cover all TDX memory regions

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Message-ID: <87a52584b67a6f68b7c9beade5da533bf722f719.camel@intel.com>
Date:   Wed, 14 Jun 2023 22:45:22 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "nik.borisov@...e.com" <nik.borisov@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "Hansen, Dave" <dave.hansen@...el.com>,
        "david@...hat.com" <david@...hat.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Luck, Tony" <tony.luck@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "Gao, Chao" <chao.gao@...el.com>,
        "Brown, Len" <len.brown@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v11 11/20] x86/virt/tdx: Fill out TDMRs to cover all TDX
 memory regions

On Wed, 2023-06-14 at 15:31 +0300, Nikolay Borisov wrote:
> 
> On 4.06.23 г. 17:27 ч., Kai Huang wrote:
> <snip>
> 
> > ---
> >   arch/x86/virt/vmx/tdx/tdx.c | 94 ++++++++++++++++++++++++++++++++++++-
> >   1 file changed, 93 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> > index 7a20c72361e7..fa9fa8bc581a 100644
> > --- a/arch/x86/virt/vmx/tdx/tdx.c
> > +++ b/arch/x86/virt/vmx/tdx/tdx.c
> > @@ -385,6 +385,93 @@ static void free_tdmr_list(struct tdmr_info_list *tdmr_list)
> >   			tdmr_list->max_tdmrs * tdmr_list->tdmr_sz);
> >   }
> >   
> > +/* Get the TDMR from the list at the given index. */
> > +static struct tdmr_info *tdmr_entry(struct tdmr_info_list *tdmr_list,
> > +				    int idx)
> > +{
> > +	int tdmr_info_offset = tdmr_list->tdmr_sz * idx;
> > +
> > +	return (void *)tdmr_list->tdmrs + tdmr_info_offset;
> 
> nit: I would just like to point that sizeof(void *) being treated as 1 
> is a gcc-specific compiler extension: 
> https://gcc.gnu.org/onlinedocs/gcc-4.4.2/gcc/Pointer-Arith.html#Pointer-Arith
> 
> I don't know if clang treats it the same way, just for the sake of 
> simplicity you might wanna change this (void *) to (char *).

Then we will need additional cast from 'char *' to 'struct tdmr_info *' I
suppose?  Not sure whether it is worth the additional cast.

And I found such 'void *' arithmetic operation is already used in other kernel
code too, e.g., below code in networking code:

	./net/rds/tcp_send.c:105:     (void *)&rm->m_inc.i_hdr + hdr_off,

and I believe there are other examples too (that I didn't spend a lot of time to
grep).

And it seems Linus also thinks "using arithmetic on 'void *' is generally
superior":

https://lore.kernel.org/lkml/CAHk-=whFKYMrF6euVvziW+drw7-yi1pYdf=uccnzJ8k09DoTXA@mail.gmail.com/t/#m983827708903c8c5bddf193343d392c9ed5af1a0

So I wouldn't worry about the Clang thing.