Re: [PATCH v20 22/28] x86/traps: Attempt to fixup exceptions in vDSO before signaling

From: Jarkko Sakkinen
Date: Tue Jun 25 2019 - 11:43:52 EST

On Wed, Apr 17, 2019 at 01:39:32PM +0300, Jarkko Sakkinen wrote:
> From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> vDSO functions can now leverage an exception fixup mechanism similar to
> kernel exception fixup. For vDSO exception fixup, the initial user is
> Intel's Software Guard Extensions (SGX), which will wrap the low-level
> transitions to/from the enclave, i.e. EENTER and ERESUME instructions,
> in a vDSO function and leverage fixup to intercept exceptions that would
> otherwise generate a signal. This allows the vDSO wrapper to return the
> fault information directly to its caller, obviating the need for SGX
> applications and libraries to juggle signal handlers.
> Attempt to fixup vDSO exceptions immediately prior to populating and
> sending signal information. Except for the delivery mechanism, an
> exception in a vDSO function should be treated like any other exception
> in userspace, e.g. any fault that is successfully handled by the kernel
> should not be directly visible to userspace.
> Although it's debatable whether or not all exceptions are of interest to
> enclaves, defer to the vDSO fixup to decide whether to do fixup or
> generate a signal. Future users of vDSO fixup, if there ever are any,
> will undoubtedly have different requirements than SGX enclaves, e.g. the
> fixup vs. signal logic can be made function specific if/when necessary.
> Suggested-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Cc: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>

I went through the vDSO changes just to revisit couple of details that I
had forgotten. Sean, if you don't mind I'd squash this and prepending

Is there any obvious reason why #PF fixup is in its own patch and the
rest are collected to the same patch? I would not find it confusing if
there was one patch per exception but really don't get this division.