[RFC PATCH 10/22] x86/fpu/xstate: Update xstate save function for supporting dynamic user xstate

[Chang S. Bae]

copy_xregs_to_kernel() used to save all user states in an invariably
sufficient buffer. When the dynamic user state is enabled, it becomes
conditional which state to be saved.

fpu->state_mask can indicate which state components are reserved to be
saved in XSAVE buffer. Use it as XSAVE's instruction mask to select states.

KVM saves xstate in guest_fpu and user_fpu. With the change, the KVM code
needs to ensure a valid fpu->state_mask before XSAVE.

No functional change until the kernel supports dynamic user states.

Signed-off-by: Chang S. Bae <chang.seok....@intel.com>
Reviewed-by: Len Brown <len.br...@intel.com>
Cc: x...@kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: k...@vger.kernel.org
---
 arch/x86/include/asm/fpu/internal.h |  3 +--
 arch/x86/kernel/fpu/core.c          |  2 +-
 arch/x86/kvm/x86.c                  | 11 ++++++++---
 3 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/fpu/internal.h 
b/arch/x86/include/asm/fpu/internal.h
index 2dfb3b6f58fc..3b03ead87a46 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -331,9 +331,8 @@ static inline void copy_kernel_to_xregs_booting(struct 
xregs_state *xstate)
 /*
  * Save processor xstate to xsave area.
  */
-static inline void copy_xregs_to_kernel(struct xregs_state *xstate)
+static inline void copy_xregs_to_kernel(struct xregs_state *xstate, u64 mask)
 {
-       u64 mask = xfeatures_mask_all;
        u32 lmask = mask;
        u32 hmask = mask >> 32;
        int err;
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index dca4961fcc36..ece6428ba85b 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -99,7 +99,7 @@ int copy_fpregs_to_fpstate(struct fpu *fpu)
        if (likely(use_xsave())) {
                struct xregs_state *xsave = &xstate->xsave;
 
-               copy_xregs_to_kernel(xsave);
+               copy_xregs_to_kernel(xsave, fpu->state_mask);
 
                /*
                 * AVX512 state is tracked here because its use is
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ecec6418ccca..a8b5f507083c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8842,15 +8842,20 @@ static int complete_emulated_mmio(struct kvm_vcpu *vcpu)
 
 static void kvm_save_current_fpu(struct fpu *fpu)
 {
+       struct fpu *src_fpu = &current->thread.fpu;
+
        /*
         * If the target FPU state is not resident in the CPU registers, just
         * memcpy() from current, else save CPU state directly to the target.
         */
-       if (test_thread_flag(TIF_NEED_FPU_LOAD))
-               memcpy(&fpu->state, &current->thread.fpu.state,
+       if (test_thread_flag(TIF_NEED_FPU_LOAD)) {
+               memcpy(&fpu->state, &src_fpu->state,
                       fpu_kernel_xstate_default_size);
-       else
+       } else {
+               if (fpu->state_mask != src_fpu->state_mask)
+                       fpu->state_mask = src_fpu->state_mask;
                copy_fpregs_to_fpstate(fpu);
+       }
 }
 
 /* Swap (qemu) user FPU context for the guest FPU context. */
-- 
2.17.1