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charset="utf-8" Switches of altp2m-s always expect a valid altp2m to be in place (and indeed altp2m_vcpu_initialise() sets the active one to be at index 0). The compiler, however, cannot know that, and hence it cannot eliminate p2m_get_altp2m()'s case of returnin (literal) NULL. If then the compiler decides to special case that code path in the caller, the dereference in instances of atomic_dec(&p2m_get_altp2m(v)->active_vcpus); can, to the code generator, appear to be NULL dereferences, leading to In function 'atomic_dec', inlined from '...' at ...: ./arch/x86/include/asm/atomic.h:182:5: error: array subscript 0 is outside = array bounds of 'int[0]' [-Werror=3Darray-bounds=3D] Aid the compiler by adding a BUG_ON() checking the return value of the problematic p2m_get_altp2m(). Since with the use of the local variable the 2nd p2m_get_altp2m() each will look questionable at the first glance (Why is the local variable not used here?), open-code the only relevant piece of p2m_get_altp2m() there. To avoid repeatedly doing these transformations, and also to limit how "bad" the open-coding really is, convert the entire operation to an inline helper, used by all three instances (and accepting the redundant BUG_ON(idx >=3D MAX_ALTP2M) in two of the three cases). Reported-by: Charles Arnold Signed-off-by: Jan Beulich Acked-by: Andrew Cooper --- a/xen/arch/x86/hvm/vmx/vmx.c +++ b/xen/arch/x86/hvm/vmx/vmx.c @@ -4128,13 +4128,7 @@ void vmx_vmexit_handler(struct cpu_user_ } } =20 - if ( idx !=3D vcpu_altp2m(v).p2midx ) - { - BUG_ON(idx >=3D MAX_ALTP2M); - atomic_dec(&p2m_get_altp2m(v)->active_vcpus); - vcpu_altp2m(v).p2midx =3D idx; - atomic_inc(&p2m_get_altp2m(v)->active_vcpus); - } + p2m_set_altp2m(v, idx); } =20 if ( unlikely(currd->arch.monitor.vmexit_enabled) ) --- a/xen/arch/x86/include/asm/p2m.h +++ b/xen/arch/x86/include/asm/p2m.h @@ -879,6 +879,26 @@ static inline struct p2m_domain *p2m_get return v->domain->arch.altp2m_p2m[index]; } =20 +/* set current alternate p2m table */ +static inline bool p2m_set_altp2m(struct vcpu *v, unsigned int idx) +{ + struct p2m_domain *orig; + + BUG_ON(idx >=3D MAX_ALTP2M); + + if ( idx =3D=3D vcpu_altp2m(v).p2midx ) + return false; + + orig =3D p2m_get_altp2m(v); + BUG_ON(!orig); + atomic_dec(&orig->active_vcpus); + + vcpu_altp2m(v).p2midx =3D idx; + atomic_inc(&v->domain->arch.altp2m_p2m[idx]->active_vcpus); + + return true; +} + /* Switch alternate p2m for a single vcpu */ bool_t p2m_switch_vcpu_altp2m_by_id(struct vcpu *v, unsigned int idx); =20 --- a/xen/arch/x86/mm/p2m.c +++ b/xen/arch/x86/mm/p2m.c @@ -1789,13 +1789,8 @@ bool_t p2m_switch_vcpu_altp2m_by_id(stru =20 if ( d->arch.altp2m_eptp[idx] !=3D mfn_x(INVALID_MFN) ) { - if ( idx !=3D vcpu_altp2m(v).p2midx ) - { - atomic_dec(&p2m_get_altp2m(v)->active_vcpus); - vcpu_altp2m(v).p2midx =3D idx; - atomic_inc(&p2m_get_altp2m(v)->active_vcpus); + if ( p2m_set_altp2m(v, idx) ) altp2m_vcpu_update_p2m(v); - } rc =3D 1; } =20 @@ -2072,13 +2067,8 @@ int p2m_switch_domain_altp2m_by_id(struc if ( d->arch.altp2m_visible_eptp[idx] !=3D mfn_x(INVALID_MFN) ) { for_each_vcpu( d, v ) - if ( idx !=3D vcpu_altp2m(v).p2midx ) - { - atomic_dec(&p2m_get_altp2m(v)->active_vcpus); - vcpu_altp2m(v).p2midx =3D idx; - atomic_inc(&p2m_get_altp2m(v)->active_vcpus); + if ( p2m_set_altp2m(v, idx) ) altp2m_vcpu_update_p2m(v); - } =20 rc =3D 0; }