Re: [Xen-devel] XSAVE/XRSTOR crash resurgence in 4.3

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.07.13 at 18:57, Ben Guthro <ben.guthro@xxxxxxxxx> wrote:
> On Tue, Jul 16, 2013 at 12:23 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>>> On 15.07.13 at 15:49, Ben Guthro <ben.guthro@xxxxxxxxx> wrote:
>>> The following output was from a WinXP SP3 guest (32bit) on a Lenovo T430:
>>>
>>> (XEN) d1v0: fip=1b773d6e9a fdp=23773d1c48 w=8
>>> (XEN) d1v0: FIP=1b773d6e9a FDP=23773d1c48 w=8
>>> (XEN) d1v1: fip=1b79e78dee fdp=230012e3b4 w=8
>>> (XEN) d1v1: FIP=1b79e78dee FDP=230012e3b4 w=8
>>> (XEN) d1v1: fip=0000:79e78dee fdp=0000:0012e3b4
>>> (XEN) d1v1: fip=0000:79e78dee fdp=0000:0012e3b4
>>> (XEN) d1v1: fip=0000:79e78dee fdp=0000:0012e3b4
>>> (XEN) d1v1: fip=0000:79e78dee fdp=0000:0012e3b4
>>> (XEN) d1v1: fip=1b79e78dee fdp=230012d528 w=8
>>> (XEN) d1v1: FIP=1b79e78dee FDP=230012d528 w=8
>>> (XEN) d1v0: fip=4500000000 fdp=4b1000000000 w=8
>>> (XEN) d1v0: FIP=4500000000 FDP=4b1000000000 w=8
>>> (XEN) d1v1: fip=1b773d6e9a fdp=23773d1c48 w=8
>>> (XEN) d1v1: FIP=1b773d6e9a FDP=23773d1c48 w=8
>>
>> Well, it should have been quite obvious (to me) that this is
>> related to xsaveopt behavior (according to my own observations
>> of cases where the selector/offset fields don't get written). Could
>> you double check whether the system(s) you see the problem on
>> support xsaveopt, and if so, simply comment out the conditionals
>> that cause it to be used?
> 
> They do support xsaveopt.
> I'll ifdef these out for tonight's test run.

And attached/below also a tentative fix.

Jan

--- a/xen/arch/x86/xstate.c
+++ b/xen/arch/x86/xstate.c
@@ -72,9 +72,24 @@ void xsave(struct vcpu *v, uint64_t mask
     if ( word_size <= 0 || !is_pv_32bit_vcpu(v) )
     {
         if ( cpu_has_xsaveopt )
+        {
+            /*
+             * xsaveopt may not write the FPU portion even when the respective
+             * mask bit is set. For the check further down to work we hence
+             * need to put the save image back into the state that it was in
+             * right after the previous xsaveopt.
+             */
+            if ( word_size > 0 &&
+                 (ptr->fpu_sse.x[FPU_WORD_SIZE_OFFSET] == 4 ||
+                  ptr->fpu_sse.x[FPU_WORD_SIZE_OFFSET] == 2) )
+            {
+                ptr->fpu_sse.fip.sel = 0;
+                ptr->fpu_sse.fdp.sel = 0;
+            }
             asm volatile ( ".byte 0x48,0x0f,0xae,0x37"
                            : "=m" (*ptr)
                            : "a" (lmask), "d" (hmask), "D" (ptr) );
+        }
         else
             asm volatile ( ".byte 0x48,0x0f,0xae,0x27"
                            : "=m" (*ptr)

Attachment: x86-xsaveopt-preserve-selectors.patch
Description: Text document

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