On Thu, May 22, 2025 at 08:10:09AM -0700, Chao Gao wrote: > From: Yang Weijiang <weijiang.yang@xxxxxxxxx> > > == Background == > > CET defines two register states: CET user, which includes user-mode control > registers, and CET supervisor, which consists of shadow-stack pointers for > privilege levels 0-2. > > Current kernels disable shadow stacks in kernel mode, making the CET > supervisor state unused and eliminating the need for context switching. > > == Problem == > > To virtualize CET for guests, KVM must accurately emulate hardware > behavior. A key challenge arises because there is no CPUID flag to indicate > that shadow stack is supported only in user mode. Therefore, KVM cannot > assume guests will not enable shadow stacks in kernel mode and must > preserve the CET supervisor state of vCPUs. > > == Solution == > > An initial proposal to manually save and restore CET supervisor states > using raw RDMSR/WRMSR in KVM was rejected due to performance concerns and > its impact on KVM's ABI. Instead, leveraging the kernel's FPU > infrastructure for context switching was favored [1]. > > The main question then became whether to enable the CET supervisor state > globally for all processes or restrict it to vCPU processes. This decision > involves a trade-off between a 24-byte XSTATE buffer waste for all non-vCPU > processes and approximately 100 lines of code complexity in the kernel [2]. > The agreed approach is to first try this optimal solution [3], i.e., > restricting the CET supervisor state to guest FPUs only and eliminating > unnecessary space waste. > > The guest-only xfeature infrastructure has already been added. Now, > introduce CET supervisor xstate support as the first guest-only feature > to prepare for the upcoming CET virtualization in KVM. > > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx> > Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx> > Reviewed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx> > Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx> Reviewed-by: John Allen <john.allen@xxxxxxx>
