i386 Transparent Paravirtualization Patch #2 Changes required to low level fault / system call code for supporting transparent paravirtualization where the kernel may be running at non-zero CPL. The testing of VM_MASK and CS together uses a neat trick which is now extended to support CPL 0,1,2 kernels. Note that assembly code must test against __ESPFIX_SS, the 16-bit stack on interrupts and exceptions. To do so at non-zero CPL requires masking off the RPL of the selector (which is a nop for the mach-default implementation). Also, there is no reason to load __KERNEL_DS into %ds or %es. __KERNEL_DS is actually quite poorly named, and is in fact the kernel stack selector. The kernel uses __USER_DS internally as both %es and %ds (since it is a flat CPL-3 accessible data segment that allows the kernel to skip reloading segments on sysexit). The reason for avoiding the __KERNEL_DS macro is it is predefined as an RPL zero segment. Diffs against: linux-2.6.13-rc6 Signed-off-by: Zachary Amsden <zach@xxxxxxxxxx> Index: linux-2.6.13/arch/i386/kernel/entry.S =================================================================== --- linux-2.6.13.orig/arch/i386/kernel/entry.S 2005-08-17 11:16:54.000000000 -0700 +++ linux-2.6.13/arch/i386/kernel/entry.S 2005-08-17 11:17:13.000000000 -0700 @@ -159,8 +159,9 @@ ret_from_intr: GET_THREAD_INFO(%ebp) movl EFLAGS(%esp), %eax # mix EFLAGS and CS movb CS(%esp), %al - testl $(VM_MASK | 3), %eax - jz resume_kernel + andl $(VM_MASK | 3), %eax + cmpl $3, %eax + jb resume_kernel ENTRY(resume_userspace) CLI # make sure we don't miss an interrupt # setting need_resched or sigpending @@ -399,10 +400,11 @@ syscall_badsys: #define UNWIND_ESPFIX_STACK \ pushl %eax; \ movl %ss, %eax; \ + MASK_RPL(%ax); \ /* see if on 16bit stack */ \ cmpw $__ESPFIX_SS, %ax; \ jne 28f; \ - movl $__KERNEL_DS, %edx; \ + movl $__USER_DS, %edx; \ movl %edx, %ds; \ movl %edx, %es; \ /* switch to 32bit stack */ \ @@ -500,7 +502,7 @@ ENTRY(simd_coprocessor_error) ENTRY(device_not_available) pushl $-1 # mark this as an int SAVE_ALL - movl %cr0, %eax + GET_CR0 testl $0x4, %eax # EM (math emulation bit) jne device_not_available_emulate preempt_stop @@ -557,6 +559,7 @@ debug_stack_correct: ENTRY(nmi) pushl %eax movl %ss, %eax + MASK_RPL(%ax) cmpw $__ESPFIX_SS, %ax popl %eax je nmi_16bit_stack Index: linux-2.6.13/include/asm-i386/mach-default/mach_asm.h =================================================================== --- linux-2.6.13.orig/include/asm-i386/mach-default/mach_asm.h 2005-08-17 11:17:09.000000000 -0700 +++ linux-2.6.13/include/asm-i386/mach-default/mach_asm.h 2005-08-17 11:17:13.000000000 -0700 @@ -7,5 +7,6 @@ #define CLI cli #define STI sti #define STI_SYSEXIT sti; sysexit +#define MASK_RPL(seg) /* nop */ #endif
