On Mon, Mar 29, 2021 at 11:43 AM Len Brown <l...@kernel.org> wrote: > > On Mon, Mar 29, 2021 at 9:33 AM Thomas Gleixner <t...@linutronix.de> wrote: > > > > I found the author of this passage, and he agreed to revise it to say this > > > was targeted primarily at VMMs. > > > > Why would this only a problem for VMMs? > > VMMs may have to emulate different hardware for different guest OS's, > and they would likely "context switch" XCR0 to achieve that. > > As switching XCR0 at run-time would confuse the heck out of user-space, > it was not imagined that a bare-metal OS would do that.
to clarify... *switching* XCR0 on context switch is slow, but perfectly legal. *changing* XCR0 during the lifetime of a process, in any of its tasks, on any of its CPUs, will confuse any software that uses xgetbv/XCR0 to calculate the size of XSAVE buffers for userspace threading. -- Len Brown, Intel Open Source Technology Center
