On Fri, Jun 21, 2024 at 12:38:21AM +0200, Paolo Bonzini wrote: > On 3/9/24 02:09, Sean Christopherson wrote: > > From: Yan Zhao <[email protected]> > > > > Ensure a full memory barrier is emitted in the VM-Exit path, as a full > > barrier is required on Intel CPUs to evict WC buffers. This will allow > > unconditionally honoring guest PAT on Intel CPUs that support self-snoop. > > > > As srcu_read_lock() is always called in the VM-Exit path and it internally > > has a smp_mb(), call smp_mb__after_srcu_read_lock() to avoid adding a > > second fence and make sure smp_mb() is called without dependency on > > implementation details of srcu_read_lock(). > > Do you really need mfence or is a locked operation enough? mfence is mb(), > not smp_mb(). > A locked operation should be enough, since the barrier here is to evict partially filled WC buffers.
" If the WC buffer is partially filled, the writes may be delayed until the next occurrence of a serializing event; such as an SFENCE or MFENCE instruction, CPUID or other serializing instruction, a read or write to uncached memory, an interrupt occurrence, or an execution of a LOCK instruction (including one with an XACQUIRE or XRELEASE prefix). " > > > + /* > > + * Call this to ensure WC buffers in guest are evicted after each VM > > + * Exit, so that the evicted WC writes can be snooped across all cpus > > + */ > > + smp_mb__after_srcu_read_lock(); >
