From 123c7793797502b222300eb710cd3873dcca41ee Mon Sep 17 00:00:00 2001 From: Andrew Cooper Date: Fri, 10 Apr 2015 11:26:18 -0400 Subject: [PATCH] VTd/dmar: Tweak how the DMAR table is clobbered Intead of clobbering DMAR -> XMAR and back, clobber to RMAD instead. This means that changing the signature does not alter the checksum, which allows the clobbering/unclobbering to be peformed atomically and idempotently, which is an advantage on the kexec path which can reenter acpi_dmar_reinstate(). This DMAR clobbering was introduced by 83904107a33c9badc34ecdd1f8ca0f9271e5e370 which claims that the dom0 VT-d driver was capable of playing with the IOMMU(s) while Xen was also using them. An alternative approach might be to leave the DMAR table alone and sprinkle some iomem_deny_access() around to forcibly prevent dom0 from playing but this is simpler. Signed-off-by: Andrew Cooper CC: Yang Zhang Acked-by: Kevin Tian --- xen/drivers/passthrough/vtd/dmar.c | 20 ++++++++++---------- 1 file changed, 10 insertions(+), 10 deletions(-) diff --git a/xen/drivers/passthrough/vtd/dmar.c b/xen/drivers/passthrough/vtd/dmar.c index 1152c3a908..18d7903ec2 100644 --- a/xen/drivers/passthrough/vtd/dmar.c +++ b/xen/drivers/passthrough/vtd/dmar.c @@ -28,6 +28,7 @@ #include #include #include +#include #include #include "dmar.h" #include "iommu.h" @@ -838,8 +839,7 @@ static int __init acpi_parse_dmar(struct acpi_table_header *table) out: /* Zap ACPI DMAR signature to prevent dom0 using vt-d HW. */ - dmar->header.signature[0] = 'X'; - dmar->header.checksum -= 'X'-'D'; + acpi_dmar_zap(); return ret; } @@ -867,18 +867,18 @@ int __init acpi_dmar_init(void) void acpi_dmar_reinstate(void) { - if ( dmar_table == NULL ) - return; - dmar_table->signature[0] = 'D'; - dmar_table->checksum += 'X'-'D'; + uint32_t sig = 0x52414d44; /* "DMAR" */ + + if ( dmar_table ) + write_atomic((uint32_t*)&dmar_table->signature[0], sig); } void acpi_dmar_zap(void) { - if ( dmar_table == NULL ) - return; - dmar_table->signature[0] = 'X'; - dmar_table->checksum -= 'X'-'D'; + uint32_t sig = 0x44414d52; /* "RMAD" - doesn't alter table checksum */ + + if ( dmar_table ) + write_atomic((uint32_t*)&dmar_table->signature[0], sig); } int platform_supports_intremap(void) -- 2.39.5