Messages in this thread | | | Date | Wed, 7 Oct 2020 00:39:27 +0300 | From | Jarkko Sakkinen <> | Subject | Re: [PATCH v39 21/24] x86/vdso: Implement a vDSO for Intel SGX enclave call |
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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
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