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i386/kvm: do not zero out segment flags if segment is unusable or not present 

This is a fix for the problem [1], where VMCB.CPL was set to 0 and interrupt
was taken on userspace stack.  The root cause lies in the specific AMD CPU
behaviour which manifests itself as unusable segment attributes on SYSRET[2].

Here in this patch flags are not touched even segment is unusable or is not
present, therefore CPL (which is stored in DPL field) should not be lost and
will be successfully restored on kvm/svm kernel side.

Also current patch should not break desired behavior described in this commit:

4cae9c9796 ("target-i386: kvm: clear unusable segments' flags in migration")

since present bit will be dropped if segment is unusable or is not present.

This is the second part of the whole fix of the corresponding problem [1],
first part is related to kvm/svm kernel side and does exactly the same:
segment attributes are not zeroed out.

[1] Message id: CAJrWOzD6Xq==b-zYCDdFLgSRMPM-NkNuTSDFEtX=7MreT45i7Q@mail.gmail.com
[2] Message id: 5d120f358612d73fc909f5bfa47e7bd082db0af0.1429841474.git.luto@kernel.org

Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
Signed-off-by: Mikhail Sennikovskii <mikhail.sennikovskii@profitbricks.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Michael Chapman <mike@very.puzzling.org>
Cc: qemu-devel@nongnu.org
Message-Id: <20170601085604.12980-1-roman.penyaev@profitbricks.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Roman Pen 2017-06-01 10:56:04 +02:00 committed by Paolo Bonzini
parent c25a67f0c3
commit d45fc087c2
1 changed files with 8 additions and 12 deletions
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20
target/i386/kvm.c
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@ -1301,18 +1301,14 @@ static void get_seg(SegmentCache *lhs, const struct kvm_segment *rhs)
lhs->selector = rhs->selector;
lhs->base = rhs->base;
lhs->limit = rhs->limit;
if (rhs->unusable) {
lhs->flags = 0;
} else {
lhs->flags = (rhs->type << DESC_TYPE_SHIFT) |
(rhs->present * DESC_P_MASK) |
(rhs->dpl << DESC_DPL_SHIFT) |
(rhs->db << DESC_B_SHIFT) |
(rhs->s * DESC_S_MASK) |
(rhs->l << DESC_L_SHIFT) |
(rhs->g * DESC_G_MASK) |
(rhs->avl * DESC_AVL_MASK);
}
lhs->flags = (rhs->type << DESC_TYPE_SHIFT) |
((rhs->present && !rhs->unusable) * DESC_P_MASK) |
(rhs->dpl << DESC_DPL_SHIFT) |
(rhs->db << DESC_B_SHIFT) |
(rhs->s * DESC_S_MASK) |
(rhs->l << DESC_L_SHIFT) |
(rhs->g * DESC_G_MASK) |
(rhs->avl * DESC_AVL_MASK);
}
static void kvm_getput_reg(__u64 *kvm_reg, target_ulong *qemu_reg, int set)