On Fri, Nov 1, 2019 at 11:38 AM Moger, Babu <Babu.Moger@xxxxxxx> wrote: > > > > On 11/1/19 1:24 PM, Andy Lutomirski wrote: > > On Fri, Nov 1, 2019 at 10:33 AM Moger, Babu <Babu.Moger@xxxxxxx> wrote: > >> > >> AMD 2nd generation EPYC processors support UMIP (User-Mode Instruction > >> Prevention) feature. The UMIP feature prevents the execution of certain > >> instructions if the Current Privilege Level (CPL) is greater than 0. > >> If any of these instructions are executed with CPL > 0 and UMIP > >> is enabled, then kernel reports a #GP exception. > >> > >> The idea is taken from articles: > >> https://lwn.net/Articles/738209/ > >> https://lwn.net/Articles/694385/ > >> > >> Enable the feature if supported on bare metal and emulate instructions > >> to return dummy values for certain cases. > > > > What are these cases? > > It is mentioned in the article https://lwn.net/Articles/738209/ > > === How does it impact applications? > > When enabled, however, UMIP will change the behavior that certain > applications expect from the operating system. For instance, programs > running on WineHQ and DOSEMU2 rely on some of these instructions to > function. Stas Sergeev found that Microsoft Windows 3.1 and dos4gw use the > instruction SMSW when running in virtual-8086 mode [4]. SGDT and SIDT can > also be used on virtual-8086 mode. > What does that have to do with your series? Your series is about enabling UMIP (or emulating UMIP -- your descriptions are quite unclear) on AMD hardware, and the hypervisor should *not* be emulating instructions to return dummy values. The *guest kernel* already knows how to emulate userspace instructions as needed. --Andy
