linux-kernel - [PATCH 08/61] KVM: x86: Warn on zero-size save state for valid CPUID 0xD.N sub-leaf

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Date:   Sat,  1 Feb 2020 10:51:25 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.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
Subject: [PATCH 08/61] KVM: x86: Warn on zero-size save state for valid CPUID 0xD.N sub-leaf

WARN if the save state size for a valid XCR0-managed sub-leaf is zero,
which would indicate a KVM or CPU bug.  Add a comment to explain why KVM
WARNs so the reader doesn't have to tease out the relevant bits from
Intel's SDM and KVM's XCR0/XSS code.

Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
---
 arch/x86/kvm/cpuid.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index fd9b29aa7abc..424dde41cb5d 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -677,10 +677,17 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
 				goto out;
 
 			do_host_cpuid(&entry[i], function, idx);
-			if (entry[i].eax == 0)
-				continue;
-			if (WARN_ON_ONCE(entry[i].ecx & 1))
+
+			/*
+			 * The @supported check above should have filtered out
+			 * invalid sub-leafs as well as sub-leafs managed by
+			 * IA32_XSS MSR.  Only XCR0-managed sub-leafs should
+			 * reach this point, and they should have a non-zero
+			 * save state size.
+			 */
+			if (WARN_ON_ONCE(!entry[i].eax || (entry[i].ecx & 1)))
 				continue;
+
 			entry[i].ecx = 0;
 			entry[i].edx = 0;
 			++*nent;
-- 
2.24.1