linux-kernel - [PATCH] KVM: x86/xcr0: Don't make XFEATURE_MASK

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Date:   Wed, 19 Jan 2022 15:04:27 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Like Xu <likexu@...cent.com>
Subject: [PATCH] KVM: x86/xcr0: Don't make XFEATURE_MASK_SSE a mandatory bit setting

From: Like Xu <likexu@...cent.com>

If "bits 1 and 2 of XCR0" are cleared and set only "bit 0 of XCR0",
the x86 hardware does work properly and the CPUID leaf of the states
size will be updated as follows:

xsetbv(XCR_XFEATURE_ENABLED_MASK, 0x2e7);
  CPUID[0d, 00]: eax=000002e7 ebx=00000a88 ecx=00000a88 edx=00000000
xsetbv(XCR_XFEATURE_ENABLED_MASK, 0x1);
  CPUID[0d, 00]: eax=000002e7 ebx=00000240 ecx=00000a88 edx=00000000

According to Intel SDM "XSETBV - Set Extended Control Register",
the truth emerges: only the "bit 0 of XCR0 (corresponding to x87 state)
must be set to 1", and in addition, it's necessary to set both bits
(XCR0[1] and XCR0[2]) to use AVX instructions".

Therefore, it's pretty safe for KVM to run a user-defined guest w/o
using AVX instructions and in that case, the kvm_update_cpuid_runtime()
cannot work as expected due to the wrong macro definition.

Remove the XFEATURE_MASK_SSE bit as part of the XFEATURE_MASK_EXTEND
and opportunistically, move it into the context of its unique user KVM.

Fixes: 56c103ec040b ("KVM: x86: Fix xsave cpuid exposing bug")
Signed-off-by: Like Xu <likexu@...cent.com>
---
 arch/x86/include/asm/fpu/xstate.h | 3 ---
 arch/x86/kvm/cpuid.h              | 8 ++++++++
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
index cd3dd170e23a..7d331a4aa2e2 100644
--- a/arch/x86/include/asm/fpu/xstate.h
+++ b/arch/x86/include/asm/fpu/xstate.h
@@ -9,9 +9,6 @@
 #include <asm/fpu/api.h>
 #include <asm/user.h>
 
-/* Bit 63 of XCR0 is reserved for future expansion */
-#define XFEATURE_MASK_EXTEND	(~(XFEATURE_MASK_FPSSE | (1ULL << 63)))
-
 #define XSTATE_CPUID		0x0000000d
 
 #define TILE_CPUID		0x0000001d
diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
index 8a770b481d9d..5e36157febb2 100644
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -8,6 +8,14 @@
 #include <asm/processor.h>
 #include <uapi/asm/kvm_para.h>
 
+/*
+ * Bit 63 of XCR0 is reserved for future expansion,
+ * and will not represent a processor state component.
+ *
+ * Only bit 0 of XCR0 (x87 state) must be set to 1.
+ */
+#define XFEATURE_MASK_EXTEND	(~(XFEATURE_MASK_FP | (1ULL << 63)))
+
 extern u32 kvm_cpu_caps[NR_KVM_CPU_CAPS] __read_mostly;
 void kvm_set_cpu_caps(void);
 
-- 
2.33.1