linux-kernel - [PATCH v2 5/8] KVM: x86: Reduce the number of emulator GPRs to '8' for 32-bit KVM

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Date:   Thu, 26 May 2022 21:08:14 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Robert Dinse <nanook@...imo.com>
Subject: [PATCH v2 5/8] KVM: x86: Reduce the number of emulator GPRs to '8'
 for 32-bit KVM

Reduce the number of GPRs emulated by 32-bit KVM from 16 to 8.  KVM does
not support emulating 64-bit mode on 32-bit host kernels, and so should
never generate accesses to R8-15.

Opportunistically use NR_EMULATOR_GPRS in rsm_load_state_{32,64}() now
that it is precise and accurate for both flavors.

Wrap the definition with full #ifdef ugliness; sadly, IS_ENABLED()
doesn't guarantee a compile-time constant as far as BUILD_BUG_ON() is
concerned.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/emulate.c     | 4 ++--
 arch/x86/kvm/kvm_emulate.h | 7 ++++---
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 5aaf1d1200af..77161f57c8d3 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -2441,7 +2441,7 @@ static int rsm_load_state_32(struct x86_emulate_ctxt *ctxt,
 	ctxt->eflags =             GET_SMSTATE(u32, smstate, 0x7ff4) | X86_EFLAGS_FIXED;
 	ctxt->_eip =               GET_SMSTATE(u32, smstate, 0x7ff0);
 
-	for (i = 0; i < 8; i++)
+	for (i = 0; i < NR_EMULATOR_GPRS; i++)
 		*reg_write(ctxt, i) = GET_SMSTATE(u32, smstate, 0x7fd0 + i * 4);
 
 	val = GET_SMSTATE(u32, smstate, 0x7fcc);
@@ -2498,7 +2498,7 @@ static int rsm_load_state_64(struct x86_emulate_ctxt *ctxt,
 	u16 selector;
 	int i, r;
 
-	for (i = 0; i < 16; i++)
+	for (i = 0; i < NR_EMULATOR_GPRS; i++)
 		*reg_write(ctxt, i) = GET_SMSTATE(u64, smstate, 0x7ff8 - i * 8);
 
 	ctxt->_eip   = GET_SMSTATE(u64, smstate, 0x7f78);
diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
index 3a65d6ea7fe6..034c845b3c63 100644
--- a/arch/x86/kvm/kvm_emulate.h
+++ b/arch/x86/kvm/kvm_emulate.h
@@ -306,11 +306,12 @@ typedef void (*fastop_t)(struct fastop *);
  * tracked/accessed via _eip, and except for RIP relative addressing, which
  * also uses _eip, RIP cannot be a register operand nor can it be an operand in
  * a ModRM or SIB byte.
- *
- * TODO: this is technically wrong for 32-bit KVM, which only supports 8 GPRs;
- * R8-R15 don't exist.
  */
+#ifdef CONFIG_X86_64
 #define NR_EMULATOR_GPRS	16
+#else
+#define NR_EMULATOR_GPRS	8
+#endif
 
 struct x86_emulate_ctxt {
 	void *vcpu;
-- 
2.36.1.255.ge46751e96f-goog