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NVMe: Add shadow doorbell support
Shadow doorbell feature was added in the NVMe spec to improve the performance of virtual devices. Typically, ringing a doorbell involves writing to an MMIO register in QEMU, which can be expensive as there will be a trap for the VM. Shadow doorbell mechanism was added for the VM to communicate with the OS when it needs to do an MMIO write, thereby avoiding it when it is not necessary. There is no performance improvement with this support in Serenity at the moment because of the block layer constraint of not batching multiple IOs. Once the command batching support is added to the block layer, shadow doorbell support can improve performance by avoiding many MMIO writes. Default to old MMIO mechanism if shadow doorbell is not supported.
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5b774f3617
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5 changed files with 121 additions and 6 deletions
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@ -28,6 +28,8 @@ struct DoorbellRegister {
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struct Doorbell {
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Memory::TypedMapping<DoorbellRegister volatile> mmio_reg;
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Memory::TypedMapping<DoorbellRegister> dbbuf_shadow;
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Memory::TypedMapping<DoorbellRegister> dbbuf_eventidx;
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};
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enum class QueueType {
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@ -62,9 +64,25 @@ public:
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protected:
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u32 process_cq();
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// Updates the shadow buffer and returns if mmio is needed
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bool update_shadow_buf(u16 new_value, u32* dbbuf, u32* ei)
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{
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u32 const old = *dbbuf;
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*dbbuf = new_value;
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AK::full_memory_barrier();
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bool need_mmio = static_cast<u16>(new_value - *ei - 1) < static_cast<u16>(new_value - old);
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return need_mmio;
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}
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void update_sq_doorbell()
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{
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m_db_regs.mmio_reg->sq_tail = m_sq_tail;
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full_memory_barrier();
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if (m_db_regs.dbbuf_shadow.paddr.is_null()
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|| update_shadow_buf(m_sq_tail, &m_db_regs.dbbuf_shadow->sq_tail, &m_db_regs.dbbuf_eventidx->sq_tail))
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m_db_regs.mmio_reg->sq_tail = m_sq_tail;
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}
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NVMeQueue(NonnullOwnPtr<Memory::Region> rw_dma_region, Memory::PhysicalPage const& rw_dma_page, u16 qid, u32 q_depth, OwnPtr<Memory::Region> cq_dma_region, OwnPtr<Memory::Region> sq_dma_region, Doorbell db_regs);
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@ -88,7 +106,10 @@ private:
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virtual void complete_current_request(u16 cmdid, u16 status) = 0;
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void update_cq_doorbell()
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{
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m_db_regs.mmio_reg->cq_head = m_cq_head;
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full_memory_barrier();
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if (m_db_regs.dbbuf_shadow.paddr.is_null()
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|| update_shadow_buf(m_cq_head, &m_db_regs.dbbuf_shadow->cq_head, &m_db_regs.dbbuf_eventidx->cq_head))
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m_db_regs.mmio_reg->cq_head = m_cq_head;
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}
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protected:
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