[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [xen master] x86: document how stub exception recovery works
commit 28154a245c2b8191854b7cafac6a69ffad4afcf3 Author: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> AuthorDate: Tue Feb 27 13:48:48 2024 +0100 Commit: Jan Beulich <jbeulich@xxxxxxxx> CommitDate: Tue Feb 27 13:48:48 2024 +0100 x86: document how stub exception recovery works Describe how it is meant to work, even if one aspect of it will only be taken care of subsequently. Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx> --- xen/arch/x86/extable.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/xen/arch/x86/extable.c b/xen/arch/x86/extable.c index 1555b9408f..474a1893f8 100644 --- a/xen/arch/x86/extable.c +++ b/xen/arch/x86/extable.c @@ -94,6 +94,22 @@ search_exception_table(const struct cpu_user_regs *regs) if ( region && region->ex ) return search_one_extable(region->ex, region->ex_end, regs->rip); + /* + * Emulation stubs (which are per-CPU) are constructed with a RET at the + * end, and are CALLed by the invoking code. + * + * An exception in the stubs may occur anywhere, so we first match any + * %rip in the correct stub, with a sanity check on %rsp too. But, an + * entry in ex_table[] needs to be compile-time constant, so we register + * the fixup address using the invoking CALL's return address. + * + * To recover, we: + * 1) Emulate a pseudo-RET to get out of the stub. We POP the return + * address off the stack(s), use it to look up the fixup address, and + * JMP there, then + * 2) Emulate a PUSH of 'token' onto the data stack to pass information + * about the exception back to the invoking code. + */ if ( regs->rip >= stub + STUB_BUF_SIZE / 2 && regs->rip < stub + STUB_BUF_SIZE && regs->rsp > (unsigned long)regs && -- generated by git-patchbot for /home/xen/git/xen.git#master
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