vp_modern_set_extended_features() currently writes all four 32-bit driver feature registers. Selecting and writing each register requires two MMIO accesses, and each access may cause a VM exit when Linux runs as a guest.
The virtio core resets a device before feature negotiation. Resetting clears all driver feature registers, so the initial finalization only needs to write through the highest nonzero register. A typical modern device uses bits 0..63 and therefore saves four MMIO accesses, or up to four VM exits, during each feature negotiation. Devices using a feature in bits 64..95 save two. A driver's validate callback may remove features and cause a second finalization without another reset. Track how many registers the previous finalization wrote and cover at least that range on the next call, writing zero where needed to clear a removed feature. Signed-off-by: Peng Hao <[email protected]> --- drivers/virtio/virtio_pci_modern_dev.c | 38 ++++++++++++++++++++++++-- include/linux/virtio_pci_modern.h | 3 ++ 2 files changed, 38 insertions(+), 3 deletions(-) diff --git a/drivers/virtio/virtio_pci_modern_dev.c b/drivers/virtio/virtio_pci_modern_dev.c index 413a8c3..631b60f 100644 --- a/drivers/virtio/virtio_pci_modern_dev.c +++ b/drivers/virtio/virtio_pci_modern_dev.c @@ -437,6 +437,11 @@ vp_modern_get_driver_extended_features(struct virtio_pci_modern_device *mdev, } EXPORT_SYMBOL_GPL(vp_modern_get_driver_extended_features); +static u32 vp_modern_features_dword(const u64 *features, unsigned int index) +{ + return features[index / 2] >> (32 * (index % 2)); +} + /* * vp_modern_set_extended_features - set features to device * @mdev: the modern virtio-pci device @@ -446,14 +451,38 @@ void vp_modern_set_extended_features(struct virtio_pci_modern_device *mdev, const u64 *features) { struct virtio_pci_common_cfg __iomem *cfg = mdev->common; + u8 required_features_dwords = VIRTIO_FEATURES_BITS / 32; + u8 features_dwords_to_write; int i; - for (i = 0; i < VIRTIO_FEATURES_BITS / 32; i++) { - u32 cur = features[i >> 1] >> (32 * (i & 1)); + /* + * Find the highest 32-bit feature register containing a negotiated + * feature. Register 0 is always written, even when no device-specific + * feature was negotiated. + */ + while (required_features_dwords > 1 && + !vp_modern_features_dword(features, + required_features_dwords - 1)) + required_features_dwords--; + /* + * vp_modern_set_status(mdev, 0) clears every driver feature register + * and sets driver_features_dwords to 0. The first finalization + * therefore only needs to write required_features_dwords. If + * finalization is repeated without a reset, also rewrite every register + * touched by the previous call so that features removed by validation + * are cleared. + */ + features_dwords_to_write = max(required_features_dwords, + mdev->driver_features_dwords); + + for (i = 0; i < features_dwords_to_write; i++) { vp_iowrite32(i, &cfg->guest_feature_select); - vp_iowrite32(cur, &cfg->guest_feature); + vp_iowrite32(vp_modern_features_dword(features, i), + &cfg->guest_feature); } + + mdev->driver_features_dwords = required_features_dwords; } EXPORT_SYMBOL_GPL(vp_modern_set_extended_features); @@ -495,6 +524,9 @@ void vp_modern_set_status(struct virtio_pci_modern_device *mdev, { struct virtio_pci_common_cfg __iomem *cfg = mdev->common; + if (!status) + mdev->driver_features_dwords = 0; + /* * Per memory-barriers.txt, wmb() is not needed to guarantee * that the cache coherent memory writes have completed diff --git a/include/linux/virtio_pci_modern.h b/include/linux/virtio_pci_modern.h index 9a3f2fc..7a5b200 100644 --- a/include/linux/virtio_pci_modern.h +++ b/include/linux/virtio_pci_modern.h @@ -27,6 +27,8 @@ * Returns the found device id or ERRNO * @dma_mask: Optional mask instead of the traditional DMA_BIT_MASK(64), * for vendor devices with DMA space address limitations + * @driver_features_dwords: Number of 32-bit driver feature registers written + * by the most recent feature finalization, or 0 after reset */ struct virtio_pci_modern_device { struct pci_dev *pci_dev; @@ -49,6 +51,7 @@ struct virtio_pci_modern_device { int (*device_id_check)(struct pci_dev *pdev); u64 dma_mask; + u8 driver_features_dwords; }; /*
