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    SubjectRe: [PATCH 1/4] x86/wakeup/sleep: Check whether the TSS GDT descriptor is empty before using it.
    On Wed, Oct 17, 2012 at 05:03:09PM -0700, H. Peter Anvin wrote:
    > On 10/17/2012 06:49 AM, Konrad Rzeszutek Wilk wrote:
    > >We check the TSS descriptor before we try to dereference it.
    > >Also fix up the value to use the #defines.
    > >
    > >Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
    > >---
    > > arch/x86/power/cpu.c | 7 +++++--
    > > 1 files changed, 5 insertions(+), 2 deletions(-)
    > >
    > >diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
    > >index 218cdb1..c17370e 100644
    > >--- a/arch/x86/power/cpu.c
    > >+++ b/arch/x86/power/cpu.c
    > >@@ -133,7 +133,9 @@ static void fix_processor_context(void)
    > > {
    > > int cpu = smp_processor_id();
    > > struct tss_struct *t = &per_cpu(init_tss, cpu);
    > >-
    > >+#ifdef CONFIG_X86_64
    > >+ struct desc_struct *desc = get_cpu_gdt_table(cpu);
    > >+#endif
    > > set_tss_desc(cpu, t); /*
    > > * This just modifies memory; should not be
    > > * necessary. But... This is necessary, because
    > >@@ -142,7 +144,8 @@ static void fix_processor_context(void)
    > > */
    > >
    > > #ifdef CONFIG_X86_64
    > >- get_cpu_gdt_table(cpu)[GDT_ENTRY_TSS].type = 9;
    > >+ if (!desc_empty(&desc[GDT_ENTRY_TSS]))
    > >+ desc[GDT_ENTRY_TSS].type = DESC_TSS;
    > >
    > > syscall_init(); /* This sets MSR_*STAR and related */
    > > #endif
    > >
    >
    > Why is this patch necessary? Presumably there is something further
    > down the line which depends on the TSS descriptor being empty, but
    > if so, what?

    Ah, so at least on Xen, that desc is marked as RO b/c it has been given
    to the hypervisor. And the hypervisor adds its own entries. Lastly on
    64-bit, the TSS for PV guests is not used - as the PV kernel and user-space
    both run in the user-space ring. So there is nothing in that entry.

    The issue I stumbled upon was just a page-fault b/c of trying to modify
    a RO region. The other way of fixing this (without knowing that the
    TSS entry is not set), was to use the functions/macros, as such:


    if (alternative_tss) {
    memcpy(&tss, &desc[GDT_ENTRY_TSS], sizeof(tss_desc));
    tss.type = DESC_TSS;
    write_gdt_entry(desc, GDT_ENTRY_TSS, &tss, DESC_TSS);
    } else {
    if (!desc_empty(&desc[GDT_ENTRY_TSS])) {
    desc[GDT_ENTRY_TSS].type = DESC_TSS;
    }
    }

    which works as well and does not throw the person from trying to figure
    out why the TSS descriptor is empty (or not).

    >
    > -hpa
    >
    > --
    > H. Peter Anvin, Intel Open Source Technology Center
    > I work for Intel. I don't speak on their behalf.


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