From: David Vrabel <david.vrabel@citrix.com>
To: xen-devel@lists.xenproject.org
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
David Vrabel <david.vrabel@citrix.com>,
Jan Beulich <jbeulich@suse.com>
Subject: [PATCHv2 1/3] x86/fpu: improve check for XSAVE* not writing FIP/FDP fields
Date: Tue, 23 Feb 2016 11:05:37 +0000 [thread overview]
Message-ID: <1456225539-9162-2-git-send-email-david.vrabel@citrix.com> (raw)
In-Reply-To: <1456225539-9162-1-git-send-email-david.vrabel@citrix.com>
The hardware may not write the FIP/FDP fields with a XSAVE*
instruction. e.g., with XSAVEOPT/XSAVES if the state hasn't changed
or on AMD CPUs when a floating point exception is not pending. We
need to identify this case so we can correctly apply the check for
whether to save/restore FCS/FDS.
By toggling FIP[63] we can turn the field into a non-canonical address
and check for this value after the XSAVE instruction.
This results in smaller code with fewer branches and is more
understandable.
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
xen/arch/x86/xstate.c | 43 +++++++++++++------------------------------
1 file changed, 13 insertions(+), 30 deletions(-)
diff --git a/xen/arch/x86/xstate.c b/xen/arch/x86/xstate.c
index 4f2fb8e..0869789 100644
--- a/xen/arch/x86/xstate.c
+++ b/xen/arch/x86/xstate.c
@@ -263,41 +263,24 @@ void xsave(struct vcpu *v, uint64_t mask)
if ( word_size <= 0 || !is_pv_32bit_vcpu(v) )
{
- typeof(ptr->fpu_sse.fip.sel) fcs = ptr->fpu_sse.fip.sel;
- typeof(ptr->fpu_sse.fdp.sel) fds = ptr->fpu_sse.fdp.sel;
+ uint64_t bad_fip;
- if ( cpu_has_xsaveopt || cpu_has_xsaves )
- {
- /*
- * XSAVEOPT/XSAVES 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;
- }
- }
+ /*
+ * FIP/FDP may not be written in some cases (e.g., if
+ * XSAVEOPT/XSAVES is used, or on AMD CPUs if an exception
+ * isn't pending).
+ *
+ * To tell if the hardware writes these fields, make the FIP
+ * field non-canonical by flipping the top bit.
+ */
+ bad_fip = ptr->fpu_sse.fip.addr ^= 1ull << 63;
XSAVE("0x48,");
- if ( !(mask & ptr->xsave_hdr.xstate_bv & XSTATE_FP) ||
- /*
- * AMD CPUs don't save/restore FDP/FIP/FOP unless an exception
- * is pending.
- */
- (!(ptr->fpu_sse.fsw & 0x0080) &&
- boot_cpu_data.x86_vendor == X86_VENDOR_AMD) )
+ /* FIP/FDP not updated? Restore the old FIP value. */
+ if ( ptr->fpu_sse.fip.addr == bad_fip )
{
- if ( (cpu_has_xsaveopt || cpu_has_xsaves) && word_size > 0 )
- {
- ptr->fpu_sse.fip.sel = fcs;
- ptr->fpu_sse.fdp.sel = fds;
- }
+ ptr->fpu_sse.fip.addr ^= 1ull << 63;
return;
}
--
2.1.4
next prev parent reply other threads:[~2016-02-23 11:05 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-23 11:05 [PATCHv2 0/3] x86: workaround inability to fully restore FPU state David Vrabel
2016-02-23 11:05 ` David Vrabel [this message]
2016-02-23 11:18 ` [PATCHv2 1/3] x86/fpu: improve check for XSAVE* not writing FIP/FDP fields Andrew Cooper
2016-02-23 11:54 ` David Vrabel
2016-02-23 14:07 ` Jan Beulich
2016-02-23 14:59 ` Jan Beulich
2016-02-23 17:42 ` David Vrabel
2016-02-24 7:51 ` Jan Beulich
2016-02-24 10:37 ` Tian, Kevin
2016-02-24 10:49 ` Jan Beulich
2016-03-18 18:23 ` Lai, Paul C
2016-02-23 11:05 ` [PATCHv2 2/3] x86/fpu: Add a per-domain field to set the width of FIP/FDP David Vrabel
2016-02-23 11:10 ` Andrew Cooper
2016-02-23 11:53 ` David Vrabel
2016-02-23 15:24 ` Jan Beulich
2016-02-23 16:27 ` David Vrabel
2016-02-23 16:39 ` Jan Beulich
2016-02-23 11:05 ` [PATCHv2 3/3] x86/hvm: add HVM_PARAM_X87_FIP_WIDTH David Vrabel
2016-02-23 11:20 ` Andrew Cooper
2016-02-24 11:51 ` Wei Liu
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