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Date:   Wed, 7 Oct 2020 00:39:27 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     x86@kernel.org, linux-sgx@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Andy Lutomirski <luto@amacapital.net>,
        Jethro Beekman <jethro@fortanix.com>,
        Cedric Xing <cedric.xing@intel.com>, akpm@linux-foundation.org,
        andriy.shevchenko@linux.intel.com, asapek@google.com, bp@alien8.de,
        chenalexchen@google.com, conradparker@google.com,
        cyhanish@google.com, dave.hansen@intel.com, haitao.huang@intel.com,
        kai.huang@intel.com, kai.svahn@intel.com, kmoy@google.com,
        ludloff@google.com, luto@kernel.org, nhorman@redhat.com,
        npmccallum@redhat.com, puiterwijk@redhat.com, rientjes@google.com,
        tglx@linutronix.de, yaozhangx@google.com, mikko.ylinen@intel.com
Subject: Re: [PATCH v39 21/24] x86/vdso: Implement a vDSO for Intel SGX
 enclave call
Message-ID: <20201006213927.GA117517@linux.intel.com>
References: <20201003045059.665934-1-jarkko.sakkinen@linux.intel.com>
 <20201003045059.665934-22-jarkko.sakkinen@linux.intel.com>
 <20201006025703.GG15803@linux.intel.com>
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Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
Precedence: bulk

On Mon, Oct 05, 2020 at 07:57:05PM -0700, Sean Christopherson wrote:
> On Sat, Oct 03, 2020 at 07:50:56AM +0300, Jarkko Sakkinen wrote:
> > +	__u16 exception_vector;
> > +	__u16 exception_error_code;
> > +	__u64 exception_addr;
> > +	__u8  reserved[24];
> 
> I also think it's a waste of space to bother with multiple reserved fields.
> 24 bytes isn't so much that it guarantees we'll never run into problems in
> the future.  But I care far less about this than I do about exit_reason.

For me the real problem is that there has not been "no brainer" basis
for any size, so a one cache line worth of data is just something that
makes sense, because would neither make much sense to have less.

I'll throw an argument to have it a bit bigger amount of reserved space
for future use.

First, there is always some amount of unknown unknowns when it comes to
run-time structures, given the evolution of microarchitectures. So yes,
some more "state" might be needed in the future.

Secondly, this is a bigger problem for the vDSO than it is for ioctl's
because we can have only one. With ioctl's, in the absolute worst case,
we can have a second version of the same ioctl.

At least 256 bytes would be probably a good number, if we want to
increase it. The reserved space zero validation that I implemented to
this version probably does not add much to the overhead anyway.

I'm not sure why care about one struct field more than making sure that
the run-time structure can stand time.

/Jarkko