KVM: x86: avoid large stack allocations in em_fxrstor

em_fxstor previously called fxstor_fixup.  Both created instances of
struct fxregs_state on the stack, which triggered the warning:

arch/x86/kvm/emulate.c:4018:12: warning: stack frame size of 1080 bytes
in function
      'em_fxrstor' [-Wframe-larger-than=]
static int em_fxrstor(struct x86_emulate_ctxt *ctxt)
           ^
with CONFIG_FRAME_WARN set to 1024.

This patch does the fixup in em_fxstor now, avoiding one additional
struct fxregs_state, and now fxstor_fixup can be removed as it has no
other call sites.

Further, the calculation for offsets into xmm_space can be shared
between em_fxstor and em_fxsave.

Signed-off-by: Nick Desaulniers <nick.desaulniers@gmail.com>
[Clean up calculation of offsets and fix it for 64-bit mode. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 0816ab2..0f0815c 100644 (file)
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -3941,6 +3941,25 @@ static int check_fxsr(struct x86_emulate_ctxt *ctxt)
 }
 
 /*
+ * Hardware doesn't save and restore XMM 0-7 without CR4.OSFXSR, but does save
+ * and restore MXCSR.
+ */
+static size_t __fxstate_size(int nregs)
+{
+       return offsetof(struct fxregs_state, xmm_space[0]) + nregs * 16;
+}
+
+static inline size_t fxstate_size(struct x86_emulate_ctxt *ctxt)
+{
+       bool cr4_osfxsr;
+       if (ctxt->mode == X86EMUL_MODE_PROT64)
+               return __fxstate_size(16);
+
+       cr4_osfxsr = ctxt->ops->get_cr(ctxt, 4) & X86_CR4_OSFXSR;
+       return __fxstate_size(cr4_osfxsr ? 8 : 0);
+}
+
+/*
  * FXSAVE and FXRSTOR have 4 different formats depending on execution mode,
  *  1) 16 bit mode
  *  2) 32 bit mode
@@ -3961,7 +3980,6 @@ static int check_fxsr(struct x86_emulate_ctxt *ctxt)
 static int em_fxsave(struct x86_emulate_ctxt *ctxt)
 {
        struct fxregs_state fx_state;
-       size_t size;
        int rc;
 
        rc = check_fxsr(ctxt);
@@ -3977,68 +3995,42 @@ static int em_fxsave(struct x86_emulate_ctxt *ctxt)
        if (rc != X86EMUL_CONTINUE)
                return rc;
 
-       if (ctxt->ops->get_cr(ctxt, 4) & X86_CR4_OSFXSR)
-               size = offsetof(struct fxregs_state, xmm_space[8 * 16/4]);
-       else
-               size = offsetof(struct fxregs_state, xmm_space[0]);
-
-       return segmented_write_std(ctxt, ctxt->memop.addr.mem, &fx_state, size);
-}
-
-static int fxrstor_fixup(struct x86_emulate_ctxt *ctxt,
-               struct fxregs_state *new)
-{
-       int rc = X86EMUL_CONTINUE;
-       struct fxregs_state old;
-
-       rc = asm_safe("fxsave %[fx]", , [fx] "+m"(old));
-       if (rc != X86EMUL_CONTINUE)
-               return rc;
-
-       /*
-        * 64 bit host will restore XMM 8-15, which is not correct on non-64
-        * bit guests.  Load the current values in order to preserve 64 bit
-        * XMMs after fxrstor.
-        */
-#ifdef CONFIG_X86_64
-       /* XXX: accessing XMM 8-15 very awkwardly */
-       memcpy(&new->xmm_space[8 * 16/4], &old.xmm_space[8 * 16/4], 8 * 16);
-#endif
-
-       /*
-        * Hardware doesn't save and restore XMM 0-7 without CR4.OSFXSR, but
-        * does save and restore MXCSR.
-        */
-       if (!(ctxt->ops->get_cr(ctxt, 4) & X86_CR4_OSFXSR))
-               memcpy(new->xmm_space, old.xmm_space, 8 * 16);
-
-       return rc;
+       return segmented_write_std(ctxt, ctxt->memop.addr.mem, &fx_state,
+                                  fxstate_size(ctxt));
 }
 
 static int em_fxrstor(struct x86_emulate_ctxt *ctxt)
 {
        struct fxregs_state fx_state;
        int rc;
+       size_t size;
 
        rc = check_fxsr(ctxt);
        if (rc != X86EMUL_CONTINUE)
                return rc;
 
-       rc = segmented_read_std(ctxt, ctxt->memop.addr.mem, &fx_state, 512);
-       if (rc != X86EMUL_CONTINUE)
-               return rc;
+       ctxt->ops->get_fpu(ctxt);
 
-       if (fx_state.mxcsr >> 16)
-               return emulate_gp(ctxt, 0);
+       size = fxstate_size(ctxt);
+       if (size < __fxstate_size(16)) {
+               rc = asm_safe("fxsave %[fx]", , [fx] "+m"(fx_state));
+               if (rc != X86EMUL_CONTINUE)
+                       goto out;
+       }
 
-       ctxt->ops->get_fpu(ctxt);
+       rc = segmented_read_std(ctxt, ctxt->memop.addr.mem, &fx_state, size);
+       if (rc != X86EMUL_CONTINUE)
+               goto out;
 
-       if (ctxt->mode < X86EMUL_MODE_PROT64)
-               rc = fxrstor_fixup(ctxt, &fx_state);
+       if (fx_state.mxcsr >> 16) {
+               rc = emulate_gp(ctxt, 0);
+               goto out;
+       }
 
        if (rc == X86EMUL_CONTINUE)
                rc = asm_safe("fxrstor %[fx]", : [fx] "m"(fx_state));
 
+out:
        ctxt->ops->put_fpu(ctxt);
 
        return rc;