Virtual Command Support (VCS) - Virtual register intended to help support virtualization of the IOMMU. Unlike an SR-IOV device where an entire device is exposed to a guest, the new model creates device instances using PASID. This requires the PASID to be a flat global space which requires the guest and host PASIDs to be the same. Only virtual IOMMUs exposed to a guest would enumerate this capability. It provides an interface to for the host to control allocation of PASIDs in a guest OS.

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/* Virtual command interface for enlightened pasid management. */
#define VCMD_CMD_ALLOC          0x1
#define VCMD_CMD_FREE           0x2
#define VCMD_VRSP_IP            0x1
#define VCMD_VRSP_SC(e)         (((e) & 0xff) >> 1)
#define VCMD_VRSP_SC_SUCCESS        0
#define VCMD_VRSP_SC_NO_PASID_AVAIL 16
#define VCMD_VRSP_SC_INVALID_PASID  16
#define VCMD_VRSP_RESULT_PASID(e)   (((e) >> 16) & 0xfffff)
#define VCMD_CMD_OPERAND(e)     ((e) << 16)

int vcmd_alloc_pasid(struct intel_iommu *iommu, u32 *pasid)
{
    unsigned long flags;
    u8 status_code;
    int ret = 0;
    u64 res;

    raw_spin_lock_irqsave(&iommu->register_lock, flags);
    dmar_writeq(iommu->reg + DMAR_VCMD_REG, VCMD_CMD_ALLOC);
    IOMMU_WAIT_OP(iommu, DMAR_VCRSP_REG, dmar_readq,
              !(res & VCMD_VRSP_IP), res);
    raw_spin_unlock_irqrestore(&iommu->register_lock, flags);

    status_code = VCMD_VRSP_SC(res);
    switch (status_code) {
    case VCMD_VRSP_SC_SUCCESS:
        *pasid = VCMD_VRSP_RESULT_PASID(res);
        break;
    case VCMD_VRSP_SC_NO_PASID_AVAIL:
        pr_info("IOMMU: %s: No PASID available\n", iommu->name);
        ret = -ENOSPC;
        break;
    default:
        ret = -ENODEV;
        pr_warn("IOMMU: %s: Unexpected error code %d\n",
            iommu->name, status_code);
    }

    return ret;
}

void vcmd_free_pasid(struct intel_iommu *iommu, u32 pasid)
{
    unsigned long flags;
    u8 status_code;
    u64 res;

    raw_spin_lock_irqsave(&iommu->register_lock, flags);
    dmar_writeq(iommu->reg + DMAR_VCMD_REG,
            VCMD_CMD_OPERAND(pasid) | VCMD_CMD_FREE);
    IOMMU_WAIT_OP(iommu, DMAR_VCRSP_REG, dmar_readq,
              !(res & VCMD_VRSP_IP), res);
    raw_spin_unlock_irqrestore(&iommu->register_lock, flags);

    status_code = VCMD_VRSP_SC(res);
    switch (status_code) {
    case VCMD_VRSP_SC_SUCCESS:
        break;
    case VCMD_VRSP_SC_INVALID_PASID:
        pr_info("IOMMU: %s: Invalid PASID\n", iommu->name);
        break;
    default:
        pr_warn("IOMMU: %s: Unexpected error code %d\n",
            iommu->name, status_code);
    }
}

流程主要分为如下两个步骤:

  1. guest分配pasid时，写vIOMMU的vcmd寄存器，此时会trap下来，host会将分配好的host paisd传给guest，这样guest与host的pasid就一样了。

  2. 当guest配置WQ Configuration register(MMIO寄存器)的PASID field时需要trap下来，hypervisor会检查guest的pasid与host的pasid是否一致，如果一致，那么hypervisor会将这个host PASID写入物理WQ Configuration register的PASID field。

参考资料:

  1. RECENT ENHANCEMENTS IN INTEL® VIRTUALIZATION TECHNOLOGY FOR DIRECTED I/O (INTEL® VT-D)
  2. Linux kernel v6.3
  3. Intel VT-d spec