AFAIK 64-bit linux has kernel in 0 and userspace in 3?<p><a href="http:&#x2F;&#x2F;wiki.osdev.org&#x2F;SYSENTER" rel="nofollow">http:&#x2F;&#x2F;wiki.osdev.org&#x2F;SYSENTER</a><p><a href="http:&#x2F;&#x2F;www.x86-64.org&#x2F;pipermail&#x2F;discuss&#x2F;2000-October&#x2F;001068.html" rel="nofollow">http:&#x2F;&#x2F;www.x86-64.org&#x2F;pipermail&#x2F;discuss&#x2F;2000-October&#x2F;001068....</a><p>I tried looking at the latest 4.2 kernel tree - but the assembler&#x2F;c-code that sets up and deals with sys-calls has been rather re-factored, so I&#x27;m not entirely adamant it&#x27;s ring 0 and ring 3 for both 32bit and 64bit - but I think so? (From a quick glance, no sections stand out as calling out ring 3 explicitly when talking about returning to user-land -- granted I didn&#x27;t do any searching over the code).