From 2b9d558ceff0e183c7bf6eae7399fd57e3438757 Mon Sep 17 00:00:00 2001
From: Nicholas Mosier <nmosier@stanford.edu>
Date: Sun, 10 Sep 2023 15:16:50 +0000
Subject: [PATCH] cpu-kvm: properly set x86 xsave header on gem5->KVM
 transition

If the XSAVE KVM capability is available (KVM_CAP_XSAVE), the X86KvmCPU
will try to set the x87 FPU + SSE state using KVM_SET_XSAVE, which
expects a buffer (struct kvm_xsave) in XSAVE area format (Vol. 1,
Sec. 13.4 of Intel x86 SDM). The original implementation of
`X86KvmCPU::updateKvmStateFPUXSave()`, however, improperly sets the
xsave header, which contains a bitmap of state components present
in the xsave area.

This patch defines `XSaveHeader` structure to model the xsave header,
which is expected directly following the legacy FPU region (defined in
the `FXSave` structure) in the xsave area. It then sets two bist in
the xsave header to indicate the presence of x86 FPU and SSE state
components.

GitHub issue: https://github.com/gem5/gem5/issues/296

Change-Id: I5c5c7925fa7f78a7b5e2adc209187deff53ac039
---
 src/arch/x86/kvm/x86_cpu.cc | 40 +++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/src/arch/x86/kvm/x86_cpu.cc b/src/arch/x86/kvm/x86_cpu.cc
index b88a96a2b9..da6e1bb9e1 100644
--- a/src/arch/x86/kvm/x86_cpu.cc
+++ b/src/arch/x86/kvm/x86_cpu.cc
@@ -42,6 +42,7 @@
 #include "arch/x86/regs/int.hh"
 #include "arch/x86/regs/msr.hh"
 #include "arch/x86/utility.hh"
+#include "base/bitunion.hh"
 #include "base/compiler.hh"
 #include "cpu/kvm/base.hh"
 #include "debug/Drain.hh"
@@ -117,6 +118,32 @@ struct GEM5_PACKED FXSave
 
 static_assert(sizeof(FXSave) == 512, "Unexpected size of FXSave");
 
+BitUnion64(XStateBV)
+    Bitfield<0> fpu;
+    Bitfield<1> sse;
+    Bitfield<2> avx;
+    Bitfield<4, 3> mpx;
+    Bitfield<7, 5> avx512;
+    Bitfield<8> pt;
+    Bitfield<9> pkru;
+    Bitfield<10> pasid;
+    Bitfield<12, 11> cet;
+    Bitfield<13> hdc;
+    Bitfield<14> uintr;
+    Bitfield<15> lbr;
+    Bitfield<16> hwp;
+    Bitfield<18, 17> amx;
+    Bitfield<63, 19> reserved;
+EndBitUnion(XStateBV)
+
+struct XSaveHeader
+{
+    XStateBV xstate_bv;
+    uint64_t reserved[7];
+};
+
+static_assert(sizeof(XSaveHeader) == 64, "Unexpected size of XSaveHeader");
+
 #define FOREACH_IREG() \
     do { \
         APPLY_IREG(rax, int_reg::Rax); \
@@ -912,6 +939,19 @@ X86KvmCPU::updateKvmStateFPUXSave()
 
     updateKvmStateFPUCommon(tc, xsave);
 
+    /**
+     * The xsave header (Vol. 1, Section 13.4.2 of the Intel Software
+     * Development Manual) directly follows the legacy xsave region
+     * (i.e., the FPU/SSE state). The first 8 bytes of the xsave header
+     * hold a state-component bitmap called xstate_bv. We need to set
+     * the state component bits corresponding to the FPU and SSE
+     * states.
+     */
+    XSaveHeader& xsave_hdr =
+      * (XSaveHeader *) ((char *) &kxsave + sizeof(FXSave));
+    xsave_hdr.xstate_bv.fpu = 1;
+    xsave_hdr.xstate_bv.sse = 1;
+
     if (tc->readMiscRegNoEffect(misc_reg::Fiseg))
         warn_once("misc_reg::Fiseg is non-zero.\n");