diff --git a/OvmfPkg/Include/Library/QemuFwCfgLib.h b/OvmfPkg/Include/Library/QemuFwCfgLib.h index 3e017d53a9..41c3817470 100644 --- a/OvmfPkg/Include/Library/QemuFwCfgLib.h +++ b/OvmfPkg/Include/Library/QemuFwCfgLib.h @@ -158,6 +158,22 @@ QemuFwCfgWriteBytes ( ); +/** + Skip bytes in the firmware configuration item. + + Increase the offset of the firmware configuration item without transferring + bytes between the item and a caller-provided buffer. Subsequent read, write + or skip operations will commence at the increased offset. + + @param[in] Size Number of bytes to skip. +**/ +VOID +EFIAPI +QemuFwCfgSkipBytes ( + IN UINTN Size + ); + + /** Reads a UINT8 firmware configuration value diff --git a/OvmfPkg/Library/QemuFwCfgLib/QemuFwCfgLib.c b/OvmfPkg/Library/QemuFwCfgLib/QemuFwCfgLib.c index 6b6b2c7726..7744873217 100644 --- a/OvmfPkg/Library/QemuFwCfgLib/QemuFwCfgLib.c +++ b/OvmfPkg/Library/QemuFwCfgLib/QemuFwCfgLib.c @@ -192,6 +192,50 @@ QemuFwCfgWriteBytes ( } +/** + Skip bytes in the firmware configuration item. + + Increase the offset of the firmware configuration item without transferring + bytes between the item and a caller-provided buffer. Subsequent read, write + or skip operations will commence at the increased offset. + + @param[in] Size Number of bytes to skip. +**/ +VOID +EFIAPI +QemuFwCfgSkipBytes ( + IN UINTN Size + ) +{ + UINTN ChunkSize; + UINT8 SkipBuffer[256]; + + if (!InternalQemuFwCfgIsAvailable ()) { + return; + } + + if (InternalQemuFwCfgDmaIsAvailable () && Size <= MAX_UINT32) { + InternalQemuFwCfgDmaBytes ((UINT32)Size, NULL, FW_CFG_DMA_CTL_SKIP); + return; + } + + // + // Emulate the skip by reading data in chunks, and throwing it away. The + // implementation below is suitable even for phases where RAM or dynamic + // allocation is not available or appropriate. It also doesn't affect the + // static data footprint for client modules. Large skips are not expected, + // therefore this fallback is not performance critical. The size of + // SkipBuffer is thought not to exert a large pressure on the stack in any + // phase. + // + while (Size > 0) { + ChunkSize = MIN (Size, sizeof SkipBuffer); + IoReadFifo8 (0x511, ChunkSize, SkipBuffer); + Size -= ChunkSize; + } +} + + /** Reads a UINT8 firmware configuration value