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authorRusty Russell <rusty@rustcorp.com.au>2007-07-26 10:41:05 -0700
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2007-07-26 11:35:17 -0700
commitf56a384e98aa81065038c4e16f39ed989ccae687 (patch)
tree923bcaabd76a88351fe5e61370143350747a90c4 /drivers/lguest
parentf8f0fdcd40449d318f8dc30c1b361b0b7f54134a (diff)
downloadop-kernel-dev-f56a384e98aa81065038c4e16f39ed989ccae687.zip
op-kernel-dev-f56a384e98aa81065038c4e16f39ed989ccae687.tar.gz
lguest: documentation VII: FIXMEs
Documentation: The FIXMEs Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'drivers/lguest')
-rw-r--r--drivers/lguest/interrupts_and_traps.c14
-rw-r--r--drivers/lguest/io.c10
-rw-r--r--drivers/lguest/lguest.c8
-rw-r--r--drivers/lguest/lguest_asm.S14
-rw-r--r--drivers/lguest/page_tables.c5
-rw-r--r--drivers/lguest/segments.c4
6 files changed, 55 insertions, 0 deletions
diff --git a/drivers/lguest/interrupts_and_traps.c b/drivers/lguest/interrupts_and_traps.c
index 3d98303..bd0091b 100644
--- a/drivers/lguest/interrupts_and_traps.c
+++ b/drivers/lguest/interrupts_and_traps.c
@@ -231,6 +231,20 @@ static int direct_trap(const struct lguest *lg,
* go direct, of course 8) */
return idt_type(trap->a, trap->b) == 0xF;
}
+/*:*/
+
+/*M:005 The Guest has the ability to turn its interrupt gates into trap gates,
+ * if it is careful. The Host will let trap gates can go directly to the
+ * Guest, but the Guest needs the interrupts atomically disabled for an
+ * interrupt gate. It can do this by pointing the trap gate at instructions
+ * within noirq_start and noirq_end, where it can safely disable interrupts. */
+
+/*M:006 The Guests do not use the sysenter (fast system call) instruction,
+ * because it's hardcoded to enter privilege level 0 and so can't go direct.
+ * It's about twice as fast as the older "int 0x80" system call, so it might
+ * still be worthwhile to handle it in the Switcher and lcall down to the
+ * Guest. The sysenter semantics are hairy tho: search for that keyword in
+ * entry.S :*/
/*H:260 When we make traps go directly into the Guest, we need to make sure
* the kernel stack is valid (ie. mapped in the page tables). Otherwise, the
diff --git a/drivers/lguest/io.c b/drivers/lguest/io.c
index da28812..ea68613 100644
--- a/drivers/lguest/io.c
+++ b/drivers/lguest/io.c
@@ -553,6 +553,16 @@ void release_all_dma(struct lguest *lg)
up_read(&lg->mm->mmap_sem);
}
+/*M:007 We only return a single DMA buffer to the Launcher, but it would be
+ * more efficient to return a pointer to the entire array of DMA buffers, which
+ * it can cache and choose one whenever it wants.
+ *
+ * Currently the Launcher uses a write to /dev/lguest, and the return value is
+ * the address of the DMA structure with the interrupt number placed in
+ * dma->used_len. If we wanted to return the entire array, we need to return
+ * the address, array size and interrupt number: this seems to require an
+ * ioctl(). :*/
+
/*L:320 This routine looks for a DMA buffer registered by the Guest on the
* given key (using the BIND_DMA hypercall). */
unsigned long get_dma_buffer(struct lguest *lg,
diff --git a/drivers/lguest/lguest.c b/drivers/lguest/lguest.c
index 7e7e9fb..6dfe568 100644
--- a/drivers/lguest/lguest.c
+++ b/drivers/lguest/lguest.c
@@ -250,6 +250,14 @@ static void irq_enable(void)
{
lguest_data.irq_enabled = X86_EFLAGS_IF;
}
+/*:*/
+/*M:003 Note that we don't check for outstanding interrupts when we re-enable
+ * them (or when we unmask an interrupt). This seems to work for the moment,
+ * since interrupts are rare and we'll just get the interrupt on the next timer
+ * tick, but when we turn on CONFIG_NO_HZ, we should revisit this. One way
+ * would be to put the "irq_enabled" field in a page by itself, and have the
+ * Host write-protect it when an interrupt comes in when irqs are disabled.
+ * There will then be a page fault as soon as interrupts are re-enabled. :*/
/*G:034
* The Interrupt Descriptor Table (IDT).
diff --git a/drivers/lguest/lguest_asm.S b/drivers/lguest/lguest_asm.S
index 3126ae9..f182c6a 100644
--- a/drivers/lguest/lguest_asm.S
+++ b/drivers/lguest/lguest_asm.S
@@ -39,6 +39,20 @@ LGUEST_PATCH(pushf, movl lguest_data+LGUEST_DATA_irq_enabled, %eax)
.global lguest_noirq_start
.global lguest_noirq_end
+/*M:004 When the Host reflects a trap or injects an interrupt into the Guest,
+ * it sets the eflags interrupt bit on the stack based on
+ * lguest_data.irq_enabled, so the Guest iret logic does the right thing when
+ * restoring it. However, when the Host sets the Guest up for direct traps,
+ * such as system calls, the processor is the one to push eflags onto the
+ * stack, and the interrupt bit will be 1 (in reality, interrupts are always
+ * enabled in the Guest).
+ *
+ * This turns out to be harmless: the only trap which should happen under Linux
+ * with interrupts disabled is Page Fault (due to our lazy mapping of vmalloc
+ * regions), which has to be reflected through the Host anyway. If another
+ * trap *does* go off when interrupts are disabled, the Guest will panic, and
+ * we'll never get to this iret! :*/
+
/*G:045 There is one final paravirt_op that the Guest implements, and glancing
* at it you can see why I left it to last. It's *cool*! It's in *assembler*!
*
diff --git a/drivers/lguest/page_tables.c b/drivers/lguest/page_tables.c
index cd047e8..b7a924a 100644
--- a/drivers/lguest/page_tables.c
+++ b/drivers/lguest/page_tables.c
@@ -15,6 +15,11 @@
#include <asm/tlbflush.h>
#include "lg.h"
+/*M:008 We hold reference to pages, which prevents them from being swapped.
+ * It'd be nice to have a callback in the "struct mm_struct" when Linux wants
+ * to swap out. If we had this, and a shrinker callback to trim PTE pages, we
+ * could probably consider launching Guests as non-root. :*/
+
/*H:300
* The Page Table Code
*
diff --git a/drivers/lguest/segments.c b/drivers/lguest/segments.c
index 4d4e5a4..f675a41 100644
--- a/drivers/lguest/segments.c
+++ b/drivers/lguest/segments.c
@@ -94,6 +94,10 @@ static void check_segment_use(struct lguest *lg, unsigned int desc)
|| lg->regs->ss / 8 == desc)
kill_guest(lg, "Removed live GDT entry %u", desc);
}
+/*:*/
+/*M:009 We wouldn't need to check for removal of in-use segments if we handled
+ * faults in the Switcher. However, it's probably not a worthwhile
+ * optimization. :*/
/*H:610 Once the GDT has been changed, we look through the changed entries and
* see if they're OK. If not, we'll call kill_guest() and the Guest will never
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