Re: [PATCH 3/5] x86/vsyscall: Add vsyscall emulation for #GP

[Sohil Mehta]

On 3/4/2026 10:31 PM, H. Peter Anvin wrote:
> On March 4, 2026 5:45:48 PM PST, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>> On 3/4/26 16:10, Sohil Mehta wrote:
>>> Vsyscall emulation has two modes of operation, specified by the
>>> vsyscall={xonly, emulate} kernel command line option. The vsyscall page
>>> is marked as execute-only in XONLY mode or read-execute in EMULATE mode.
>>
>> Is it really "marked as execute only"? We don't have a real execute-only
>> paging permission on x86, the closest we've got is memory marked with a
>> pkey that's got the AccessDisable bit set.
>>
>> I think it's _called_ execute-only because the kernel makes it behave
>> like execute-only memory when it's handling the page fault. But I dobn't
>> think it is super accurate to say it is "marked" as execute-only.
>>

Sorry about the wording. I should have looked at map_vsyscall() more
carefully. As Peter said the page is not even present in XONLY mode and
in EMULATE mode the PTE is marked as _USR but __NX.

How about using "behaves" instead of marked? Or I could use "emulated"
if you prefer that.

So the above paragraph would be:

Vsyscall emulation has two modes of operation, specified by the
vsyscall={xonly, emulate} kernel command line option. The vsyscall page
behaves as execute-only in XONLY mode and as read-execute in EMULATE mode.