Re: [PATCH v4 00/75] x86: SEV-ES Guest Support

[Peter Zijlstra]

On Tue, Jul 14, 2020 at 02:08:02PM +0200, Joerg Roedel wrote:
> The #VC entry code now tries to pretend that the #VC handler does not
> use an IST stack by switching to the task stack if entered from
> user-mode or the SYSCALL entry path. When it is entered from
> kernel-mode it is doing its best to switch back to the interrupted
> stack. This is only possible if it is entered from a known and safe
> kernel stack (e.g. not the entry stack). If the previous stack is not
> safe to use the #VC handler switches to a fall-back stack and calls a
> special handler function which, as of now, just panics the system. For
> now this is safe as #VC exceptions only happen at know places which
> use a safe stack.
> 
> The use of the fall-back stack is necessary so that the special
> handler function can safely raise nested #VC exceptions, for
> example to print a panic message.

Can we get some more words -- preferably in actual code comments, on
when exactly #VC happens?

Because the only thing I remember is that #VC could happen on any memop,
but I also have vague memories of that being a later extention.