Hey Andy, On Wed, Jan 17, 2018 at 10:10:23AM -0800, Andy Lutomirski wrote: > On Wed, Jan 17, 2018 at 1:18 AM, Joerg Roedel <joro@xxxxxxxxxx> wrote: > > Just read up on vm86 mode control transfers and the stack layout then. > > Looks like I need to check for eflags.vm=1 and copy four more registers > > from/to the entry stack. Thanks for pointing that out. > > You could just copy those slots unconditionally. After all, you're > slowing down entries by an epic amount due to writing CR3 on with PCID > off, so four words copied should be entirely lost in the noise. OTOH, > checking for VM86 mode is just a single bt against EFLAGS. > > With the modern (rewritten a year or two ago by Brian Gerst) vm86 > code, all the slots (those actually in pt_regs) are in the same > location regardless of whether we're in VM86 mode or not, but we're > still fiddling with the bottom of the stack. Since you're controlling > the switch to the kernel thread stack, you can easily just write the > frame to the correct location, so you should not need to context > switch sp1 -- you can do it sanely and leave sp1 as the actual bottom > of the kernel stack no matter what. In fact, you could probably avoid > context switching sp0, either, which would be a nice cleanup. I am not sure what you mean by "not context switching sp0/sp1" ... > So I recommend the following. Keep sp0 as the bottom of the sysenter > stack no matter what. Then do: > > bt $X86_EFLAGS_VM_BIT > jc .Lfrom_vm_\@ > > push 5 regs to real stack, starting at four-word offset (so they're in > the right place) > update %esp > ... > .Lupdate_esp_\@ > > .Lfrom_vm_\@: > push 9 regs to real stack, starting at the bottom > jmp .Lupdate_esp_\@ > > Does that seem reasonable? It's arguably much nicer than what we have > now. But that looks like a good idea. Having a consistent stack with and without vm86 is certainly a nice cleanup. Regards, Joerg -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
